Semiconductor Device and Electronic Apparatus Having the Same

ABSTRACT

With an offset circuit including transistors of the same conductivity type, offset of an input signal is performed. Then, the input signal after the offset is supplied to a logic circuit including transistors of the same conductivity type as that of the offset circuit, thereby H and L levels of the input signal can be shifted at the same time. Further, since the offset circuit and the logic circuit are formed using the transistors of the same conductivity type, a display device can be manufactured at a low cost.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/692,893, filed Jan. 25, 2010, now allowed, which is a continuation ofU.S. application Ser. No. 12/239,852, filed Sep. 29, 2008, now U.S. Pat.No. 7,663,404, which is a continuation of U.S. application Ser. No.11/752,546, filed May 23, 2007, now U.S. Pat. No. 7,443,202, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2006-155460 on Jun. 2, 2006, all of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a drivingmethod of a semiconductor device. In addition, the present inventionrelates to a display device having a semiconductor device, inparticular, relates to a liquid crystal display device having asemiconductor device and an electronic apparatus having the liquidcrystal display device.

2. Description of the Related Art

In recent years, display devices such as a liquid crystal display deviceand a light-emitting device have been actively developed in accordancewith increase in large display devices such as liquid crystaltelevisions. In particular, a technique in which a pixel circuit and adriver circuit including a shift register circuit or the like(hereinafter referred to as an internal circuit) are formed over thesame substrate by using transistors formed of a noncrystallinesemiconductor over an insulator, which contributes largely to reductionin power consumption and cost, has been actively developed. The internalcircuit formed over the insulator is connected via an FPC or the like toa controller IC or the like (hereinafter referred to as an externalcircuit) disposed outside the insulator, and an operation thereof iscontrolled.

As the internal circuit formed over the insulator, various levelshifters have been invented (see Reference 1: Japanese Published PatentApplication No. 2001-257581, and Reference 2: Japanese Published PatentApplication No. 2002-118458)

SUMMARY OF THE INVENTION

Each level shifter described in References 1 and 2 cannot shift levelsof an input signal on negative and positive poser source sides at thesame time. That is, in the case of shifting the levels of an inputsignal on the negative and positive power source sides, a level shifterfor shifting the level of an input signal on the negative power sourceside and a level shifter for shifting the level of an input signal onthe positive power source side have been required.

In view of the problem, it is an object of the present invention toprovide a level shifter capable of simultaneously shifting levels of aninput signal on a negative power source side and on a positive powersource side, a semiconductor device and a display device such as aliquid crystal display device each having such a level shifter, and anelectronic apparatus having the display device.

In accordance with one feature of the present invention, a firstcapacitor, a second capacitor, a first transistor, a second transistor,a third transistor, and a fourth transistor are included, and a firstelectrode of the first capacitor is electrically connected to a thirdwiring, a first electrode of the second capacitor is electricallyconnected to a fourth wiring, a gate of the first transistor iselectrically connected to a second electrode of the first capacitor, afirst terminal of the first transistor is electrically connected to asecond wiring, a second terminal of the first transistor is electricallyconnected to a second electrode of the second capacitor, a gate of thesecond transistor is electrically connected to the second electrode ofthe second capacitor, a first terminal of the second transistor iselectrically connected to the second wiring, a second terminal of thesecond transistor is electrically connected to the second electrode ofthe first capacitor, a gate of the third transistor is electricallyconnected to the second electrode of the second capacitor, a firstterminal of the third transistor is electrically connected to the secondwiring, a second terminal of the third transistor is electricallyconnected to a fifth wiring, a gate and a first terminal of the fourthtransistor are electrically connected to a first wiring, and a secondterminal of the fourth transistor is electrically connected to the fifthwiring.

Note that the first to fourth transistors may be transistors of the sameconductivity type. In the case where the first to fourth transistors areP-channel transistors, a potential of the first wiring may be higherthan a potential of the second wiring. In the case where the first tofourth transistors are N-channel transistors, the potential of the firstwiring may be lower than the potential of the second wiring.

In accordance with one feature of the present invention, a firstcapacitor, a second capacitor, a first transistor, a second transistor,a third transistor, a fourth transistor, a fifth transistor, and a sixthtransistor are included, and a first electrode of the first capacitor iselectrically connected to a third wiring, a first electrode of thesecond capacitor is electrically connected to a fourth wiring, a gate ofthe first transistor is electrically connected to a second electrode ofthe first capacitor, a first terminal of the first transistor iselectrically connected to a second wiring, a second terminal of thefirst transistor is electrically connected to a second electrode of thesecond capacitor, a gate of the second transistor is electricallyconnected to the second electrode of the second capacitor, a firstterminal of the second transistor is electrically connected to thesecond wiring, a second terminal of the second transistor iselectrically connected to the second electrode of the first capacitor, agate of the third transistor is electrically connected to the secondelectrode of the second capacitor, a first terminal of the thirdtransistor is electrically connected to the second wiring, a secondterminal of the third transistor is electrically connected to a fifthwiring, a gate and a first terminal of the fourth transistor areelectrically connected to a first wiring, a second terminal of thefourth transistor is electrically connected to the fifth wiring, a gateof the fifth transistor is electrically connected to the secondelectrode of the first capacitor, a first terminal of the fifthtransistor is electrically connected to the second wiring, a secondterminal of the fifth transistor is electrically connected to a sixthwiring, a gate and a first terminal of the sixth transistor areelectrically connected to the first wiring, and a second terminal of thesixth transistor is electrically connected to the sixth wiring.

Note that the first to sixth transistors may be transistors of the sameconductivity type. In the case where the first to sixth transistors areP-channel transistors, a potential of the first wiring may be higherthan a potential of the second wiring. In the case where the first tosixth transistors are N-channel transistors, the potential of the firstwiring may be lower than the potential of the second wiring.

In accordance with one feature of the present invention, a firstcapacitor, a second capacitor, a first transistor, a second transistor,a third transistor, a fourth transistor, a fifth transistor, and a sixthtransistor are included, and a first electrode of the first capacitor iselectrically connected to a third wiring, a first electrode of thesecond capacitor is electrically connected to a fourth wiring, a gate ofthe first transistor is electrically connected to a second electrode ofthe first capacitor, a first terminal of the first transistor iselectrically connected to a second wiring, a second terminal of thefirst transistor is electrically connected to a second electrode of thesecond capacitor, a gate of the second transistor is electricallyconnected to the second electrode of the second capacitor, a firstterminal of the second transistor is electrically connected to thesecond wiring, a second terminal of the second transistor iselectrically connected to the second electrode of the first capacitor, agate of the third transistor is electrically connected to the secondelectrode of the second capacitor, a first terminal of the thirdtransistor is electrically connected to the second wiring, a gate and afirst terminal of the fourth transistor are electrically connected to afirst wiring, a second terminal of the fourth transistor is electricallyconnected to a second terminal of the third transistor, a gate of thefifth transistor is electrically connected to the second electrode ofthe second capacitor, a first terminal of the fifth transistor iselectrically connected to the second wiring, a second terminal of thefifth transistor is electrically connected to a fifth wiring, a gate ofthe sixth transistor is electrically connected to the second terminal ofthe third transistor and the second terminal of the fourth transistor, afirst terminal of the sixth transistor is electrically connected to thefirst wiring, and a second terminal of the sixth transistor iselectrically connected to the fifth wiring.

Note that the first to sixth transistors may be transistors of the sameconductivity type. In the case where the first to sixth transistors areP-channel transistors, a potential of the first wiring may be higherthan a potential of the second wiring. In the case where the first tosixth transistors are N-channel transistors, the potential of the firstwiring may be lower than the potential of the second wiring.

In accordance with one feature of a liquid crystal display device of thepresent invention, a pixel including a liquid crystal element and adriver circuit are included, the driver circuit includes a firstcapacitor, a second capacitor, a first transistor, a second transistor,a third transistor, and a fourth transistor, and a first electrode ofthe first capacitor is electrically connected to a third wiring, a firstelectrode of the second capacitor is electrically connected to a fourthwiring, a gate of the first transistor is electrically connected to asecond electrode of the first capacitor, a first terminal of the firsttransistor is electrically connected to a second wiring, a secondterminal of the first transistor is electrically connected to a secondelectrode of the second capacitor, a gate of the second transistor iselectrically connected to the second electrode of the second capacitor,a first terminal of the second transistor is electrically connected tothe second wiring, a second terminal of the second transistor iselectrically connected to the second electrode of the first capacitor, agate of the third transistor is electrically connected to the secondelectrode of the second capacitor, a first terminal of the thirdtransistor is electrically connected to the second wiring, a secondterminal of the third transistor is electrically connected to a fifthwiring, a gate and a first terminal of the fourth transistor areelectrically connected to a first wiring, and a second terminal of thefourth transistor is electrically connected to the fifth wiring.

Note that the first to fourth transistors may be transistors of the sameconductivity type. In the case where the first to fourth transistors areP-channel transistors, a potential of the first wiring may be higherthan a potential of the second wiring. In the case where the first tofourth transistors are N-channel transistors, the potential of the firstwiring may be lower than the potential of the second wiring.

In accordance with one feature of a liquid crystal display device of thepresent invention, a pixel including a liquid crystal element and adriver circuit are included, the driver circuit includes a firstcapacitor, a second capacitor, a first transistor, a second transistor,a third transistor, a fourth transistor, a fifth transistor, and a sixthtransistor, and a first electrode of the first capacitor is electricallyconnected to a third wiring, a first electrode of the second capacitoris electrically connected to a fourth wiring, a gate of the firsttransistor is electrically connected to a second electrode of the firstcapacitor, a first terminal of the first transistor is electricallyconnected to a second wiring, a second terminal of the first transistoris electrically connected to a second electrode of the second capacitor,a gate of the second transistor is electrically connected to the secondelectrode of the second capacitor, a first terminal of the secondtransistor is electrically connected to the second wiring, a secondterminal of the second transistor is electrically connected to thesecond electrode of the first capacitor, a gate of the third transistoris electrically connected to the second electrode of the secondcapacitor, a first terminal of the third transistor is electricallyconnected to the second wiring, a second terminal of the thirdtransistor is electrically connected to a fifth wiring, a gate and afirst terminal of the fourth transistor are electrically connected to afirst wiring, a second terminal of the fourth transistor is electricallyconnected to the fifth wiring, a gate of the fifth transistor iselectrically connected to the second electrode of the first capacitor, afirst terminal of the fifth transistor is electrically connected to thesecond wiring, a second terminal of the fifth transistor is electricallyconnected to a sixth wiring, a gate and a first terminal of the sixthtransistor are electrically connected to the first wiring, and a secondterminal of the sixth transistor is electrically connected to the sixthwiring.

Note that the first to sixth transistors may be transistors of the sameconductivity type. In the case where the first to sixth transistors areP-channel transistors, a potential of the first wiring may be higherthan a potential of the second wiring. In the case where the first tosixth transistors are N-channel transistors, the potential of the firstwiring may be lower than the potential of the second wiring.

In accordance with one feature of a liquid crystal display device of thepresent invention, a pixel including a liquid crystal element and adriver circuit are included, the driver circuit includes a firstcapacitor, a second capacitor, a first transistor, a second transistor,a third transistor, a fourth transistor, a fifth transistor, and a sixthtransistor, and a first electrode of the first capacitor is electricallyconnected to a third wiring, a first electrode of the second capacitoris electrically connected to a fourth wiring, a gate of the firsttransistor is electrically connected to a second electrode of the firstcapacitor, a first terminal of the first transistor is electricallyconnected to a second wiring, a second terminal of the first transistoris electrically connected to a second electrode of the second capacitor,a gate of the second transistor is electrically connected to the secondelectrode of the second capacitor, a first terminal of the secondtransistor is electrically connected to the second wiring, a secondterminal of the second transistor is electrically connected to thesecond electrode of the first capacitor, a gate of the third transistoris electrically connected to the second electrode of the secondcapacitor, a first terminal of the third transistor is electricallyconnected to the second wiring, a gate and a first terminal of thefourth transistor are electrically connected to a first wiring, a secondterminal of the fourth transistor is electrically connected to a secondterminal of the third transistor, a gate of the fifth transistor iselectrically connected to the second electrode of the second capacitor,a first terminal of the fifth transistor is electrically connected tothe second wiring, a second terminal of the fifth transistor iselectrically connected to a fifth wiring, a gate of the sixth transistoris electrically connected to the second terminal of the third transistorand the second terminal of the fourth transistor, a first terminal ofthe sixth transistor is electrically connected to the first wiring, anda second terminal of the sixth transistor is electrically connected tothe fifth wiring.

Note that the first to sixth transistors may be transistors of the sameconductivity type. In the case where the first to sixth transistors areP-channel transistors, a potential of the first wiring may be higherthan a potential of the second wiring. In the case where the first tosixth transistors are N-channel transistors, the potential of the firstwiring may be lower than the potential of the second wiring.

Note also that a switch in the present invention may be any switch suchas an electrical switch or a mechanical switch. That is, it may beanything as long as it can control a current flow. For example, atransistor, a diode (e.g., a PN diode, a PIN diode, a Schottky diode, ora diode-connected transistor), a thyristor, or a logic circuitconfigured with them may be used. Therefore, in the case of using atransistor as a switch, a polarity (conductivity type) of the switch,which operates just as a switch, is not particularly limited. However,when less off-current is preferred, it is preferable to use a transistorof a polarity with less off-current. As an example of the transistorwith less off-current, a transistor provided with an LDD region, atransistor with a multi-gate structure, and the like can be given.Further, it is preferable that an N-channel transistor be used in thecase where a potential of a source terminal of the transistor operatingas the switch is closer to a low-potential side power source (e.g., Vss,GND, or 0 V), and a P-channel transistor be used in the case where thepotential of the source terminal of the transistor operating as theswitch is closer to a high-potential side power source (e.g., Vdd). Thisis because an absolute value of a gate-source voltage of the transistorcan be increased so that the transistor can operate easily as a switch.

Note that a CMOS switch may also be used by using both N-channel andP-channel switches. The CMOS switch can operate easily as a switchbecause a current can flow when either one of the P-channel andN-channel switches is turned on. For example, regardless of whether avoltage of an input signal to the switch is high or low, a voltage canbe outputted appropriately. Further, since a voltage amplitude of asignal for turning on or off the switch can be reduced, powerconsumption can be reduced.

Note that in the case where a transistor is used as a switch, there arean input terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal (a gate terminal) of controlling electricalconduction. On the other hand, in the case where a diode is used as aswitch, there is not necessarily a terminal of controlling electricalconduction; therefore, the number of wirings for controlling terminalscan be reduced.

Note that in the present invention, being “connected” includeselectrical connection, functional connection, and direct connection.Therefore, each structure disclosed in the present invention may includean object in addition to a predetermined connection. For example, atleast one of elements which allow electrical connection (e.g., a switch,a transistor, a capacitor, an inductor, a resistor, or a diode) may alsobe disposed between a certain portion and another certain portion.Further, at least one of circuits which allow functional connection(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converting circuit such as a DA converting circuit, anAD converting circuit, or a gamma-correction circuit; a potential-levelconverting circuit such as a power source circuit, e.g., a voltagestep-up circuit or a voltage step-down circuit, or a level shiftercircuit of changing a potential level of an Hi signal or a Low signal; avoltage source; a current source; a switching circuit; an amplifiercircuit such as a circuit capable of increasing an signal amplitude, thecurrent amount, or the like, such as an operation amplifier, adifferential amplifier circuit, a source follower circuit, or a buffercircuit; a signal generating circuit; a memory circuit; or a controllingcircuit) may also be disposed therebetween. Alternatively, withoutinterposing another element or circuit therebetween, direct connectionmay be performed.

The case only including the case where connection is performed withoutinterposing any other element or circuit is described as being “directlyconnected”. Further, the case described as being “electricallyconnected” includes the case of electrical connection (i.e., the case ofconnection with another element interposed therebetween), the case offunctional connection (i.e., the case of connection with another circuitinterposed therebetween), and the case of direct connection (i.e., thecase of connection without interposing another element or circuittherebetween).

Note that a display element, a display device, a light-emitting element,or a light-emitting device may be of various modes or may includevarious elements. For example, as the display element, the displaydevice, the light-emitting element, or the light-emitting device, adisplay medium of which contrast changes by an electromagnetic functioncan be applied, such as an EL element (e.g., an organic EL element, aninorganic EL element, or an EL element containing both organic andinorganic materials), an electron-emissive element, a liquid crystalelement, an electronic ink, a grating light valve (GLV), a plasmadisplay (PDP), a digital micromirror device (DMD), a piezoceramicdisplay, and a carbon nanotube. Note that, display devices using an ELelement include an EL display; display devices using anelectron-emissive element include a Field Emission Display (FED), aSurface-conduction Electron-emitter Display (SED), and the like; displaydevices using a liquid crystal element include a liquid crystal display,a transmissive liquid crystal display, a semi-transmissive liquidcrystal display, and a reflective liquid crystal display; and displaydevices using an electronic ink include an electronic paper.

Note that in the present invention, as a transistor, a transistor of anymode can be used. Therefore, the usable kind of a transistor is notparticularly limited. Thus, for example, a thin film transistor (TFT) orthe like including a non-single crystalline semiconductor film typifiedby non-crystalline silicon or polycrystalline silicon can be used. As aresult of this, for example, manufacturing can be performed at a lowmanufacturing temperature, at a low cost, over a large substrate, orover a transparent substrate, and the transistor can transmit light.Further, a transistor formed using a semiconductor substrate or an SOIsubstrate, a MOS transistor, a junction type transistor, a bipolartransistor, or the like can also be used. As a result of this, forexample, transistors with less variation can be manufactured,transistors having a high current supplying capability can bemanufactured, transistors with a small size can be manufactured, or acircuit with less power consumption can be formed. Further, a transistorincluding a compound semiconductor such as ZnO, a-InGaZnO, SiGe, orGaAs, a thin film transistor formed by forming a thin film of thetransistor, or the like can also be used. As a result of this, forexample, manufacturing can be performed at a low manufacturingtemperature, manufacturing can be performed at room temperature, and atransistor can be formed directly on a substrate which is low in heatresistance, such as a plastic substrate or a film substrate. Further, atransistor formed by an ink jet method or a printing method, or the likecan also be used. As a result of this, for example, manufacturing can beperformed at room temperature, manufacturing can be performed with a lowdegree of vacuum, or manufacturing can be performed with a largesubstrate. In addition, manufacturing can also be performed without amask (a reticle), so that a layout of a transistor can be easilychanged. Further, a transistor including an organic semiconductor or acarbon nanotube, or another transistor can also be used. As a result ofthis, a transistor can be formed over a substrate capable of being bent.Note that the non-single crystalline semiconductor film may containhydrogen or halogen. Further, the kind of a substrate over which atransistor is disposed is not particularly limited, and varioussubstrates can be used. Therefore, for example, a transistor can beprovided for a single crystalline substrate, an SOI substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a paper substrate, acellophane substrate, a stone substrate, a stainless steel substrate, asubstrate having stainless steel foil, or the like. Further, atransistor may be formed over a substrate, and then the transistor maybe transferred to another substrate, thereby disposing over the anothersubstrate. By using these substrates, for example, a transistor havinggood properties can be formed, a transistor with less power consumptioncan be formed, a device which is not broken so easily can be formed, orheat resistance can be provided.

A structure of a transistor can employ various modes, and is not limitedto a certain structure. For example, a multi-gate structure includingtwo or more gate electrodes can be employed. In the case of themulti-gate structure, since channel regions are connected in series, itseems like a structure in which a plurality of transistors are connectedin series. In the case of the multi-gate structure, for example,off-current can be reduced, reliability can be improved by increasingthe withstand voltage of the transistor, and a drain-source current isnot changed so much even if a drain-source voltage is changed whenoperating in the saturation region so that flat properties can beachieved. Further, a structure in which gate electrodes are providedabove and below a channel may also be employed. By employing thestructure in which gate electrodes are provided above and below achannel, a channel region is increased, so that, for example, a currentvalue can be increased and an S value can be reduced because a depletionlayer is easily formed. The structure in which gate electrodes areprovided above and below a channel seems like a structure in which aplurality of transistors are connected in parallel. Further, anystructure of the following may also be employed: a structure in which agate electrode is disposed above a channel; a structure in which a gateelectrode is disposed below a channel; a staggered structure; and aninversely staggered structure. A channel region may be divided into aplurality of regions, and the divided channel regions may be connectedin parallel or in series. Further, a source or drain electrode may beoverlapped with a channel (or a part thereof). By employing thestructure in which a source or drain electrode is overlapped with achannel (or a part thereof), operational instability due to chargeaccumulation in a part of a channel can be prevented. Further, an LDDregion may also be provided. By providing an LDD region, for example,off-current can be reduced, reliability can be improved by increasingthe withstand voltage of the transistor, and a drain-source current isnot changed so much even if a drain-source voltage is changed whenoperating in the saturation region so that flat properties can beachieved.

Note that, various types of transistors may be employed as thetransistor in the present invention, and the transistor can be formedover various types of substrates. Therefore, all of circuits may beprovided for any substrate such as a glass substrate, a plasticsubstrate, a single crystalline substrate, or an SOI substrate. Byforming all of the circuits over the same substrate, for example, thenumber of component parts can be reduced to reduce cost, and the numberof connections to the circuit components can be reduced to improvereliability. Alternatively, parts of the circuits may be formed over asubstrate and the other parts of the circuits may be formed over anothersubstrate. That is, not all of the circuits are required to be formedover the same substrate. For example, parts of the circuits may beformed with transistors over a glass substrate, and the other parts ofthe circuits may be formed using a single crystalline substrate and anIC chip thereof may be connected by COG (Chip On Glass) so as to bedisposed over the glass substrate. Alternatively, the IC chip may beconnected to the glass substrate by TAB (Tape Automated Bonding) or aprinted wiring board. In this manner, by forming parts of the circuitsover the same substrate, for example, the number of component parts canbe reduced to reduce cost, or the number of connections to the circuitcomponents can be reduced to improve reliability. In addition, byforming a portion with a high driving voltage or a portion with highdriving frequency, which would consume large power, over a differentsubstrate, increase of power consumption can be prevented.

Note also that one pixel corresponds to one element which can controlbrightness in the present invention. For example, one pixel correspondsto one color element and brightness is expressed with the one colorelement. Therefore, in that case, in the case of a color display devicehaving color elements of R (Red), G (Green), and B (Blue), a minimumunit of an image is composed of three pixels of an R pixel, a G pixel,and a B pixel. Note that the color elements are not limited to threecolors, and color elements with more than three colors may be employed.For example, RGBW (W means white), or RGB plus at least one of yellow,cyan, magenta, emerald green, and vermilion may also be employed.Further, each similar color to at least one of RGB may also be added;for example, R, G, B1, and B2 may be employed. Although both of B1 andB2 are color blue, they are slightly different in frequency. Byemploying these color elements, for example, a display which is moretrue to life can be performed, and power consumption can be reduced.Alternatively, as another example, in the case of controlling brightnessof one color element by using a plurality of regions, one regioncorresponds to one pixel. Therefore, in that case, one color element iscomposed of a plurality of pixels. Further, in that case, a region areawhich contributes to a display may be different depending on each pixel.In addition, a viewing angle may also be widened by slightly changingeach signal supplied to each of the plurality of areas for controllingbrightness provided per color element, that is, each of the plurality ofthe pixels forming one color element. Note that the description “onepixel (for three colors)” denotes the case where three pixels of R, G,and B are considered as one pixel. The description “one pixel (for onecolor)” denotes the case where a plurality of pixels, which are providedfor one color element, are collectively considered as one pixel.

Note that in the present invention, pixels may be disposed (arranged) inmatrix. Here, the case where pixels are disposed (arranged) in matrixincludes the case where pixels are disposed in line or in jagged line ina longitudinal direction or a lateral direction. Therefore, in the casewhere a full-color display is performed using color elements of threecolors (e.g., RGB), stripe arrangement and delta arrangement of dots ofthree color elements are included. Furthermore, Bayer arrangement isincluded. Note that the color elements are not limited to three colors,and color elements with more than three colors may be employed. Forexample, RGBW (W means white), or RGB plus at least one of yellow, cyan,magenta, and the like may also be employed. Further, a display regionarea may be different depending on a dot of a color element. Thereby,for example, power consumption can be reduced and life of a displaydevice can be prolonged.

Note that a transistor is a three-terminal element including a gate, adrain, and a source, in which a channel region is formed between a drainregion and a source region. Current can be supplied through the drainregion, the channel region, and the source region. Since the source andthe drain are changed depending on a structure, an operationalcondition, or the like of a transistor, it is difficult to specify thesource and the drain. Therefore, in the present invention, regionsserving as a source and a drain are not necessarily referred to as asource and a drain; for example, they are referred to as a firstterminal and a second terminal, respectively.

Note that a transistor may also be an element having at least threeterminals including a base, an emitter, and a collector. Similarly inthis case, the emitter and the collector may be referred to as a firstterminal and a second terminal.

Note that a gate means the whole of a gate electrode and a gate wiring(also called a gate line, a gate signal line, or the like) or a partthereof. A gate electrode means a part of a conductive layer, which isoverlapped with a semiconductor forming a channel region, an LDD(Lightly Doped Drain) region, and the like with a gate insulating filminterposed therebetween. A gate wiring means a wiring for connectinggate electrodes of pixels or for connecting a gate electrode to anotherwiring.

It is to be noted that there is also a portion which serves both as agate electrode and a gate wiring. Such a portion can be called either agate electrode or a gate wiring. That is, there is also a region where agate electrode and a gate wiring cannot be clearly distinguished. Forexample, in the case where a channel region is provided so as to overlapwith an extended wiring, the overlapping region serves as a gateelectrode as well as a gate wiring. Therefore, such a region can becalled either a gate electrode or a gate wiring.

Further, a region formed of the same material as a gate electrode andconnected to the gate electrode can also be called a gate electrode.Similarly, a region formed of the same material as a gate wiring andconnected to the gate wiring can also be called a gate wiring. To beexact, such a region is, in some cases, not overlapped with a channelregion or does not have a function of connecting to another gateelectrode. However, due to a manufacturing process, there is a regionformed of the same material as a gate electrode or a gate wiring and isconnected to the gate electrode or the gate wiring. Therefore, such aregion can also be called either a gate electrode or a gate wiring.

Further, for example, in a multi-gate transistor, a gate electrode ofone transistor and a gate electrode of another transistor are connectedby a conductive film formed of the same material as each gate electrodein many cases. Such a region, which is a region for connecting gateelectrodes to each other, may be called a gate wiring; however, it canalso be called a gate electrode since a multi-gate transistor can alsobe considered as one transistor. That is, a region which is formed ofthe same material as and connected to a gate electrode or a gate wiringcan also be called a gate electrode or a gate wiring. Further, forexample, a conductive film of a portion for connecting a gate electrodeto a gate wiring can also be called either a gate electrode or a gatewiring.

Note that a gate terminal means a part of a region of a gate electrodeor a region electrically connected to a gate electrode.

A source means the whole of a source region, a source electrode, and asource wiring (also called a source line, a source signal line, or thelike) or a part thereof. A source region means a semiconductor regioncontaining a P-type impurity (e.g., boron or gallium) or an N-typeimpurity (e.g., phosphorus or arsenic) at a high concentration.Therefore, a region containing a P-type impurity or an N-type impurityat a low concentration, i.e., an LDD (Lightly Doped Drain) region is notincluded in a source region. A source electrode means a part of aconductive layer, which is formed of a different material from a sourceregion and is electrically connected to the source region. A sourceelectrode may, however, include a source region. A source wiring means awiring for connecting source electrodes of pixels or for connecting asource electrode to another wiring.

It is to be noted that there is also a portion which serves both as asource electrode and a source wiring. Such a portion can be calledeither a source electrode or a source wiring. That is, there is also aregion where a source electrode and a source wiring cannot be clearlydistinguished. For example, in the case where a source region isprovided so as to overlap with an extending source wiring, theoverlapping region serves as a source electrode as well as a sourcewiring. Therefore, such a region can be called either a source electrodeor a source wiring.

Further, a region formed of the same material as a source electrode andconnected to the source electrode or a portion for connecting sourceelectrodes can also be called a source electrode. Further, a portionwhich is overlapped with a source region can also be called a sourceelectrode. Similarly, a region formed of the same material as a sourcewiring and connected to the source wiring can also be called a sourcewiring. To be exact, such a region has, in some cases, a function ofconnecting to another source electrode. However, due to a manufacturingprocess, there is a region formed of the same material as a sourceelectrode or a source wiring and is connected to the source electrode orthe source wiring. Therefore, such a region can also be called either asource electrode or a source wiring.

Further, for example, a part of a conductive film, which connects asource electrode and a source wiring, can also be called either a sourceelectrode or a source wiring.

Note that a source terminal means a part of a region of a source region,a source electrode, or a region electrically connected to a sourceelectrode.

A drain is similar to the source.

Note that in the present invention, a semiconductor device means adevice including a circuit including a semiconductor element (e.g., atransistor or a diode). Further, a semiconductor device may correspondto any device which functions by utilizing semiconductorcharacteristics.

A display device corresponds to a device including a display element(e.g., a liquid crystal element or a light-emitting element). Note thata display device may correspond to a display panel itself in which aplurality of pixels including a display element such as a liquid crystalelement or an EL element and a peripheral driver circuit for driving thepixels are formed over one substrate. Further, a display device mayinclude a peripheral driver circuit provided over a substrate by wirebonding or a bump, a so-called Chip-On-Glass (COG). Moreover, a displaydevice may include a device provided with a flexible printed circuit(FPC) or a printed wiring board (PWB) (e.g., an IC, a resistor, acapacitor, an inductor, or a transistor). Further, a display device mayinclude an optical sheet such as a polarizing plate or a phasedifferential plate. Further, a display device may include a back lightunit (which may include a light guide plate, a prism sheet, a diffusionsheet, a reflection sheet, or a light source such as an LED or acold-cathode tube).

Further, a light-emitting device corresponds to, in particular, adisplay device including a self-luminous display element such as an ELelement or an element used for an FED. A liquid crystal display devicecorresponds to a display device including a liquid crystal element.

In the present invention, a description that one object is formed on orover the other object does not necessarily mean that the one object isin direct contact with the other object. The description may include thecase where the two objects are not in direct contact with each other,that is, the case where another object is interposed therebetween.Therefore, for example, when it is described that a layer B is formed on(or over) a layer A, it means either case where the layer B is formed onand in direct contact with the layer A, or where another layer (e.g., alayer C or a layer D) is formed on and in direct contact with the layerA and the layer B is formed on and in direct contact with the layer C orD. Similarly, when it is described that one object is formed above theother object, it does not necessarily mean that the one object is on andin direct contact with the other object, and another object may beinterposed therebetween. Therefore, for example, when it is describedthat a layer B is formed above a layer A, it means either case where thelayer B is formed on and in direct contact with the layer A, or whereanother layer (e.g., a layer C or a layer D) is formed on and in directcontact with the layer A and the layer B is formed on and in directcontact with the layer C or D. Similarly, when it is described that oneobject is formed below or under the other object, it means either casewhere they are in direct contact with each other or not in contact witheach other.

In accordance with the present invention, a display device including alevel shifter capable of shifting levels of an input signal on anegative power source side and on a positive power source sidesimultaneously can be provided. Further, the display device of thepresent invention can be formed using transistors of the sameconductivity type, thereby a display device at a low cost can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are diagrams each showing a structure of a level shifterof the present invention.

FIGS. 2A and 2B are diagrams each showing a timing chart of a levelshifter of the present invention.

FIGS. 3A to 3C are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 4A and 4B are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 5A to 5C are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 6A and 6B are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 7A to 7C are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 8A and 8B are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 9A to 9C are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIGS. 10A and 10B are diagrams each showing a structure of an offsetcircuit of a level shifter of the present invention.

FIG. 11 is a diagram showing a structure of an offset circuit of a levelshifter of the present invention.

FIG. 12 is a diagram showing a structure of an offset circuit of a levelshifter of the present invention.

FIG. 13 is a diagram showing a structure of an offset circuit of a levelshifter of the present invention.

FIG. 14 is a diagram showing a structure of an offset circuit of a levelshifter of the present invention.

FIG. 15 is a diagram showing a structure of a level shifter of thepresent invention.

FIGS. 16A to 16C are diagrams each showing a timing chart of a levelshifter of the present invention.

FIG. 17 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 18 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 19 is a diagram showing a structure of a level shifter of thepresent invention.

FIGS. 20A to 20C are diagrams each showing a timing chart of a levelshifter of the present invention.

FIG. 21 is a diagram showing a structure of a level shifter of thepresent invention.

FIGS. 22A to 22D are diagrams each showing a timing chart of a levelshifter of the present invention.

FIG. 23 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 24 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 25 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 26 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 27 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 28 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 29 is a diagram showing a structure of a level shifter of thepresent invention.

FIGS. 30A to 30C are diagrams each showing a timing chart of a levelshifter of the present invention.

FIG. 31 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 32 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 33 is a diagram showing a structure of a level shifter of thepresent invention.

FIGS. 34A to 34C are diagrams each showing a timing chart of a levelshifter of the present invention.

FIG. 35 is a diagram showing a structure of a level shifter of thepresent invention.

FIGS. 36A to 36D are diagrams each showing a timing chart of a levelshifter of the present invention.

FIG. 37 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 38 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 39 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 40 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 41 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 42 is a diagram showing a structure of a level shifter of thepresent invention.

FIG. 43 is a layout diagram of a level shifter of the present invention.

FIG. 44 is a layout diagram of a level shifter of the present invention.

FIG. 45 is a layout diagram of a level shifter of the present invention.

FIG. 46 is a layout diagram of a level shifter of the present invention.

FIGS. 47A and 47B are layout diagrams of each level shifter of thepresent invention.

FIGS. 48A and 48B are diagrams each showing a cross-sectional diagram ofa pixel of the present invention.

FIG. 49 is a cross-sectional diagram of a pixel of the presentinvention.

FIGS. 50A and 50B are diagrams each showing a cross-sectional diagram ofa pixel of the present invention.

FIGS. 51A to 51C are diagrams each showing a cross-sectional diagram ofa pixel of the present invention.

FIGS. 52A and 52B are diagrams each showing a cross-sectional diagram ofa pixel of the present invention.

FIGS. 53A to 53C are diagrams each showing a display module of thepresent invention.

FIG. 54 is a diagram showing a display module of the present invention.

FIGS. 55A to 55D are diagrams for showing application of an electronicapparatus of the present invention.

FIGS. 56A and 56B are diagrams for showing application of an electronicapparatus of the present invention.

FIG. 57 is a diagram for showing application of an electronic apparatusof the present invention.

FIGS. 58A and 58B are diagrams for showing application of an electronicapparatus of the present invention.

FIG. 59 is a diagram for showing application of an electronic apparatusof the present invention.

FIG. 60 is a diagram for showing application of an electronic apparatusof the present invention.

FIG. 61 is a diagram for showing application of an electronic apparatusof the present invention.

FIGS. 62A and 62B are diagrams each showing a structure of a displaypanel of the present invention.

FIGS. 63A and 63B are diagrams respectively showing a structure of adisplay panel and a configuration of an EL pixel of the presentinvention.

FIG. 64A is a diagram showing a structure of a display panel and FIGS.64B and 64C are diagrams each showing a configuration of an EL pixel ofthe present invention.

FIGS. 65A and 65B are diagrams respectively showing a structure of adisplay panel and a configuration of a liquid crystal pixel of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention are describedwith reference to the accompanying drawings. The present invention can,however, be carried out with many different modes and it is easilyunderstood by those skilled in the art that modes and details can bemodified in various ways without departing from the purpose and thescope of the present invention. Therefore, the present invention shouldnot be interpreted as being limited to the description of the embodimentmodes. Note that in structures of the present invention describedhereinafter, reference numerals are used in common, and detaileddescription of the same portions or portions having similar functions isnot repeated.

Embodiment Mode 1

This embodiment mode describes a basic structure of a level shifter ofthe present invention with reference to FIG. 1A.

A level shifter shown in FIG. 1A includes circuits 101 and 102.

As shown in the level shifter in FIG. 1A, the circuit 101 is connectedto wirings 103, 104, 105, 106, 107, and 108. The circuit 102 isconnected to the wirings 103, 104, 107, 108, and 109.

Note that a positive power source VDD and a negative power source VSSare supplied to the wiring 103 and the wiring 104. The power sourcepotential VDD is higher than the power source potential VSS.

Signals (hereinafter also called input signals) are supplied to thewirings 105 and 106. The circuit 101 is controlled by the signalssupplied to the wirings 105 and 106.

To the wirings 107 and 108, signals (hereinafter also called offsetsignals) from the circuit 101 are supplied. The circuit 102 iscontrolled by the signals supplied to the wirings 107 and 108.

To the wiring 109, a signal (hereinafter also called an output signal)from the circuit 102 is supplied.

Each signal supplied to the wirings 105 and 106 is a binary digitalsignal. The potential of the digital signal in the case of an H signal(hereinafter also called an H level) is VH and in the case of an Lsignal (hereinafter also called an L level) is VL. The potential VH islower than the power source potential VDD and is higher than thepotential VL. The potential VL is higher than the power source potentialVSS and is lower than the potential VH. That is, the relationship amongthe power source potentials and the signals is as follows: power sourcepotential VDD>potential VH>potential VL>power source potential VSS.

Next, an operation of the level shifter shown in FIG. 1A is describedwith reference to a timing chart in FIG. 2A. In the timing chart of FIG.2A, the signals of the wirings 105, 107, and 108 are shown. Although notshown in the drawing, a signal of which H and L levels are reverse tothose of the signal of the wiring 105 corresponds to the signal of thewiring 106.

Here, the circuit 101 has a function of offset operation. Specifically,the circuit 101 offsets input signals supplied to the wirings 105 and106 and supplies offset signals to the wirings 107 and 108. Each offsetsignal has the same timing as (or the reverse timing to) the signalsupplied to the wiring 105, and has an amplitude voltage which is almostequivalent to the signal supplied to the wiring 105. A potential of theoffset signal supplied to the wiring 107 is shifted toward the H sidewith respect to the input signal. A potential of the offset signalsupplied to the wiring 108 is shifted toward the L side with respect tothe input signal. Note that the circuit 101 is also called an offsetcircuit.

Therefore, as shown in the timing chart in FIG. 2A, the timing and theamplitude voltage of the signal of the wiring 107 are almost the same asthose of the signal of the wiring 105, and the potential thereof isshifted toward the H side. Specifically, as for the signal of the wiring107, a potential of an L signal is VDD and a potential of an H signal is(VDD+(VH−VL)). That is, the amplitude voltage of the signal of thewiring 107 is (VH−VL), which is almost the same as an amplitude voltageof the signal of the wiring 105.

Similarly to the signal of the wiring 107, the timing and the amplitudevoltage of the signal of the wiring 108 are almost the same as those ofthe signal of the wiring 105, and the potential thereof is shiftedtoward the L side. Specifically, as for the signal of the wiring 108, apotential of an L signal is VSS and a potential of an H signal is(VSS+(VH−VL)). That is, the amplitude voltage of the signal of thewiring 108 is (VH−VL), which is almost the same as the amplitude voltageof the signal of the wiring 105.

Note that as described above, the signal of the wiring 107 and thesignal of the wiring 108 may be signals of which H and L signals arereverse to those of the signal of the wiring 105.

Alternatively, as shown in FIG. 2B, as for the signal of the wiring 107,the potential of the L signal may be (VDD−(VH−VL)) and the potential ofthe H signal may be VDD. Further, as for the signal of the wiring 108,the potential of the L signal may be (VSS−(VH−VL)). Even in the caseshown in FIG. 2B, either of the amplitude voltage of the signal of thewiring 107 or the amplitude voltage of the signal of the wiring 108 is(VH−VL), which is almost the same as the amplitude voltage of the signalof the wiring 105.

Note also that the signal of the wiring 107 and the signal of the wiring108 shown in FIG. 2B may also be signals of which H and L signals arereverse to those of the signal of the wiring 105.

Here, the circuit 102 is a logic circuit such as an inverter, a NANDcircuit, or a NOR circuit. Specifically, the circuit 102 is controlledby an offset signal, and supplies an output signal to the wiring 109.The potential of the output signal is equal to the power sourcepotential VDD in the case of an H signal and is equal to the powersource potential VSS in the case of an L signal.

Note that since the amplitude voltages of the signals of the wiring 107and the wiring 108 are small, through current of the circuit 102 isreduced to achieve low power consumption. Further, since the amplitudevoltages of the inputted signals of the circuit 102 are small, noise canbe reduced.

As described above, the level shifter of the present invention achievesa function as a level shifter by driving a logic circuit with an offsetsignal. Further, in the level shifter of the present invention, lowpower consumption and low noise can be realized. Further, in accordancewith the level shifter of the present invention, an H level and an Llevel of an input signal can be shifted simultaneously with one levelshifter.

Note that as shown in FIG. 1B, the circuit 102 may also be controlledonly by a signal (the signal of the wiring 107) with the potential ofeach signal of the wirings 105 and 106 shifted toward an H side. In thecase where the circuit 102 is controlled only by the signal of thewiring 107, the power source potential VSS is not necessarily suppliedto the circuit 101.

Similarly, as shown in FIG. 1C, the circuit 102 may also be controlledonly by a signal (the signal of the wiring 108) with the potential ofeach signal of the wirings 105 and 106 shifted toward an L side. In thecase where the circuit 102 is controlled only by the signal of thewiring 108, the power source potential VDD is not necessarily suppliedto the circuit 101.

By controlling the circuit 102 only by the signal of the wiring 107 orthe signal of the wiring 108, the circuit 101 in each of level shiftersshown in FIGS. 1B and 1C can employ a simple structure.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 2

This embodiment mode describes structure examples of the circuit 101(the offset circuit) included in the level shifter described inEmbodiment Mode 1. Note that in this embodiment mode, description ismade on the case where each signal supplied to the wirings 105 and 106is supplied to the wiring 107 after the potential is shifted toward an Hside while the timing is not changed (or is reversed) and the amplitudevoltage is not changed so much.

Note that in this embodiment mode and Embodiment Mode 1, referencenumerals are used in common, and detailed description of the sameportions and portions having similar functions is not repeated.

First, a structure example of an offset circuit is described withreference to FIG. 3A.

An offset circuit shown in FIG. 3A includes a capacitor 301, a capacitor302, a transistor 303, and a transistor 304.

As shown in the offset circuit in FIG. 3A, a first electrode of thecapacitor 301 is connected to the wiring 105. A first electrode of thecapacitor 302 is connected to the wiring 106. A gate of the transistor303 is connected to a second electrode of the capacitor 301, a firstterminal of the transistor 303 is connected to the wiring 103, and asecond terminal of the transistor 303 is connected to a second electrodeof the capacitor 301. A gate of the transistor 304 is connected to thesecond electrode of the capacitor 302, a first terminal of thetransistor 304 is connected to the wiring 103, and a second terminal ofthe transistor 304 is connected to the second electrode of the capacitor302. Note that a connection point of the second electrode of thecapacitor 301, the gate of the transistor 303, and the second terminalof the transistor 304 is denoted by a node N31. A connection point ofthe second electrode of the capacitor 302, the second terminal of thetransistor 303, and the gate of the transistor 304 is denoted by a nodeN32. Either one of the node N31 or the node N32 is connected to thewiring 107 shown in FIG. 1A.

Next, operations of the offset circuit shown in FIG. 3A are describedwith reference to FIGS. 3B and 3C.

FIG. 3B shows an operation of the offset circuit shown in FIG. 3A when asignal of the wiring 105 is changed from an H signal to an L signal anda signal of the wiring 106 is changed from an L signal to an H signal.

FIG. 3C shows an operation of the offset circuit shown in FIG. 3A whenthe signal of the wiring 105 is changed from the L signal to the Hsignal and the signal of the wiring 106 is changed from the H signal tothe L signal. That is, the offset circuit shown in FIG. 3A repeats theoperation shown in FIG. 3B and the operation shown in FIG. 3C at acertain timing. Note here that the operation shown in FIG. 3B is denotedby a first operation and the operation shown in FIG. 3C is denoted by asecond operation.

Note that a value of (VH−VL) is equal to or larger than each thresholdvoltage of the transistors 303 and 304.

First, the first operation of the offset circuit shown in FIG. 3A isdescribed with reference to FIG. 3B. Note here that an initial potentialof the node N32 is VDD.

In an initial state, the capacitor 302 holds a potential difference(VDD−VL) between a potential VL (the L signal) of the wiring 106 and thepotential VDD of the node N32. Then, when the potential of the wiring106 is changed from VL to VH, the potential of the node N32 becomes(VDD+(VH−VL)) by capacitive coupling of the capacitor 302. Consequently,the transistor 304 is turned on.

By turning on the transistor 304, the power source potential VDD issupplied to the node N31, so that the potential of the node N31 becomesVDD. Consequently, the capacitor 301 holds the potential difference(VDD−VL) between the potential VL (the L signal) of the wiring 105 andthe potential VDD of the node N31. In addition, the transistor 303 isturned off.

By turning off the transistor 303, the node N32 becomes a floating stateto keep the potential (VDD+(VH−VL)).

Next, the second operation of the offset circuit shown in FIG. 3A isdescribed with reference to FIG. 3C.

As described above, (VDD−VL) is held in the capacitor 301 by the firstoperation. Then, when the potential of the wiring 105 is changed from VLto VH, the potential of the node N31 becomes (VDD+(VH−VL)) by capacitivecoupling of the capacitor 301. Consequently, the transistor 303 isturned on.

By turning on the transistor 303, the power source potential VDD issupplied to the node N32, so that the potential of the node N32 becomesVDD. Consequently, the capacitor 302 holds the potential difference(VDD−VL) between the potential VL (the L signal) of the wiring 106 andthe potential VDD of the node N32. In addition, the transistor 304 isturned off.

By turning off the transistor 304, the node N31 becomes the floatingstate to keep the potential (VDD+(VH−VL)).

Here, functions of the capacitors 301 and 302 and the transistors 303and 304 are described respectively.

First, the capacitor 301 holds the potential difference between thepotential VL of the wiring 105 and the potential VDD of the node N31 inthe first operation. Then, in the second operation, the capacitor 301increases the potential of the node N31 in accordance with increase ofpotential of the wiring 105 by capacitive coupling.

The capacitor 302 holds the potential difference between the potentialVL of the wiring 106 and the potential VDD of the node N32 in the secondoperation. Then, in the first operation, the capacitor 302 increases thepotential of the node N32 in accordance with increase of potential ofthe wiring 106 by capacitive coupling.

The transistor 303 functions as a switch of selecting whether the wiring103 and the node N32 are connected or not depending on the potential ofthe node N31. In addition, in the first operation, the transistor 303 isturned off so that the node N32 becomes the floating state. In thesecond operation, the transistor 303 is turned on so that the powersource potential VDD is supplied to the node N32.

The transistor 304 functions as a switch of selecting whether the wiring103 and the node N31 are connected or not depending on the potential ofthe node N32. In addition, in the first operation, the transistor 304 isturned on so that the power source potential VDD is supplied to the nodeN31. In the second operation, the transistor 304 is turned off so thatthe node N31 becomes the floating state.

By the first and second operations described above, the offset circuitshown in FIG. 3A operates as follows: In the first operation, the powersource potential VDD is supplied to the node N31, and the node N32 isbrought into the floating state to keep the potential (VDD+(VH−VL)); andin the second operation, the node N31 is brought into the floating stateto keep the potential (VDD+(VH−VL)), and the power source potential VDDis supplied to the node N32.

Therefore, as for a signal generated from the offset circuit shown inFIG. 3A, an H signal is (VDD+(VH−VL)) and an L signal is VDD. That is,the offset circuit shown in FIG. 3A can generate a signal based on thepower source potential VDD.

Note that as each of the potential of the node N32 in the firstoperation and the potential of the node N31 in the second operation,(VDD+(VH−VL)) is kept. In practice, however, each of the potential ofthe node N32 in the first operation and the potential of the node N31 inthe second operation becomes lower than (VDD+(VH−VL)) due to wiringcapacitance, parasitic capacitance, or the like. Therefore, in order toreduce such an effect of the wiring capacitance or the parasiticcapacitance, each capacitance of the capacitors 301 and 302 may be setto be sufficiently larger than the wiring capacitance or the parasiticcapacitance.

When the node N31 is connected to the wiring 107 shown in FIG. 1A, asignal of which H and L levels are the same as those of the signalsupplied to the wiring 105 can be supplied to the wiring 107. Similarly,when the node N32 is connected to the wiring 107 shown in FIG. 1A, asignal of which H and L levels are reverse to those of the signalsupplied to the wiring 105 can be supplied to the wiring 107.

In the case where the node N31 is connected to the wiring 107, it ispreferable that the capacitance of the capacitor 302 be smaller thanthat of the capacitor 301. This is because, as described above, since itis described above that the capacitance of the capacitor 301 may be setto be sufficiently larger than the wiring capacitance or the parasiticcapacitance, the potential of the node N32 is not necessarily(VDD+(VH−VL)) as long as the transistor 304 can be turned on. Thus thecapacitance of the capacitor 302 can be smaller than that of thecapacitor 301, thereby an element region of the capacitor 302 can bereduced.

In the case where the node N32 is connected to the wiring 107, it ispreferable that the capacitance of the capacitor 301 be smaller thanthat of the capacitor 302 because of the similar reason to the casewhere the node N31 is connected to the wiring 107.

Note that each of the capacitors 301 and 302 may employ a structure inwhich an insulating layer is interposed between two electrode layers. Byemploying the structure in which an insulating layer is interposedbetween two electrode layers for each of the capacitors 301 and 302, thecapacitors 301 and 302 can keep the capacitance constant regardless ofan applied voltage, so that the level shifter of the present inventioncan be stably operated.

Further, it is preferable that an insulating layer of each of thecapacitors 301 and 302 be a gate insulating film. This is because thecapacitors 301 and 302 can obtain capacitance effectively since a gateinsulating film is generally thinner than other insulating films (e.g.,an interlayer film or a planarizing film).

Note also that a MOS capacitor may be employed as each of the capacitors301 and 302. FIG. 4A shows a structure in the case where MOS capacitorsare employed as the capacitors 301 and 302. In an offset circuit shownin FIG. 4A, an N-channel transistor 401 is used instead of the capacitor301 and an N-channel transistor 402 is used instead of the capacitor302. Note here that a gate of the transistor 401 is connected to thenode N31, and first and second terminals of the transistor 401 areconnected to the wiring 105. This is because the transistor 401 canoperate as a capacitor since the potential of the node N31 is higherthan that of the wiring 105 to turn on the transistor 401 so that achannel is formed in a channel region of the transistor 401. Similarly,a gate of the transistor 402 is connected to the node N32, and first andsecond terminals of the transistor 402 are connected to the wiring 106.This is because the transistor 402 can operate as a capacitor since thepotential of the node N32 is higher than that of the wiring 106 to turnon the transistor 402 so that a channel is formed in a channel region ofthe transistor 402.

Further, as shown in FIG. 4B, a P-channel transistor may also be used aseach capacitor. In an offset circuit shown in FIG. 4B, a P-channeltransistor 403 is used instead of the capacitor 301 and a P-channeltransistor 404 is used instead of the capacitor 302. Note here that agate of the transistor 403 is connected to the wiring 105, and first andsecond terminals of the transistor 403 are connected to the node N31.This is because the transistor 403 can operate as a capacitor since thepotential of the node N31 is higher than that of the wiring 105 to turnon the transistor 403 so that a channel is formed in a channel region ofthe transistor 403. Similarly, a gate of the transistor 404 is connectedto the wiring 106, and first and second terminals of the transistor 404are connected to the node N32. This is because the transistor 404 canoperate as a capacitor since the potential of the node N32 is higherthan that of the wiring 106 to turn on the transistor 404 so that achannel is formed in a channel region of the transistor 404.

Note that as described above, in FIG. 4A, in the case where the node N31is connected to the wiring 107, it is preferable that the channel region(L×W: L is channel length and W is channel width) of the transistor 402be smaller than that of the transistor 401. In the case where the nodeN32 is connected to the wiring 107, it is preferable that the channelregion of the transistor 402 be larger than that of the transistor 401.

Similarly, in FIG. 4B, in the case where the node N31 is connected tothe wiring 107, it is preferable that the channel region (L×W: L ischannel length and W is channel width) of the transistor 404 be smallerthan that of the transistor 403. In the case where the node N32 isconnected to the wiring 107, it is preferable that the channel region ofthe transistor 404 be larger than that of the transistor 403.

Here, although the offset circuit shown in FIG. 3A includes theN-channel transistors and the capacitors, it may include P-channeltransistors and capacitors. FIG. 5A shows an offset circuit includingP-channel transistors and capacitors.

An offset circuit shown in FIG. 5A includes the capacitor 301, thecapacitor 302, a transistor 501, and a transistor 502.

Note that the transistors 501 and 502 correspond to and have similarfunctions to the transistors 303 and 304 in FIG. 3A. Nodes N51 and N52correspond to the nodes N31 and N32 in FIG. 3A.

As shown in the offset circuit in FIG. 5A, a first electrode of thecapacitor 301 is connected to the wiring 105. A first electrode of thecapacitor 302 is connected to the wiring 106. A gate of the transistor501 is connected to a second electrode of the capacitor 301, a firstterminal of the transistor 501 is connected to the wiring 103, and asecond terminal of the transistor 501 is connected to a second electrodeof the capacitor 302. A gate of the transistor 502 is connected to thesecond electrode of the capacitor 302, a first terminal of thetransistor 502 is connected to the wiring 103, and a second terminal ofthe transistor 502 is connected to the second electrode of the capacitor301. Note that a connection point of the second electrode of thecapacitor 301, the gate of the transistor 501, and the second terminalof the transistor 502 is denoted by the node N51. A connection point ofthe second electrode of the capacitor 302, the second terminal of thetransistor 501, and the gate of the transistor 502 is denoted by thenode N52. Either one of the node N51 or the node N52 is connected to thewiring 107 shown in FIG. 1A.

Next, operations of the offset circuit shown in FIG. 5A are describedwith reference to FIGS. 5B and 5C.

FIG. 5B shows an operation of the offset circuit shown in FIG. 5A when asignal of the wiring 105 is changed from an H signal to an L signal anda signal of the wiring 106 is changed from an L signal to an H signal.

FIG. 5C shows an operation of the offset circuit shown in FIG. 5A whenthe signal of the wiring 105 is changed from the L signal to the Hsignal and the signal of the wiring 106 is changed from the H signal tothe L signal. That is, the offset circuit shown in FIG. 5A repeats theoperation shown in FIG. 5B and the operation shown in FIG. 5C at acertain timing. Note here that the operation shown in FIG. 5B is denotedby a first operation and the operation shown in FIG. 5C is denoted by asecond operation.

First, the first operation of the offset circuit shown in FIG. 5A isdescribed with reference to FIG. 5B. Note here that an initial potentialof the node N51 is VDD.

In an initial state, the capacitor 301 holds a potential difference(VDD−VH) between a potential VH (the H signal) of the wiring 105 and thepotential VDD of the node N51. Then, when the potential of the wiring105 is changed from VH to VL, the potential of the node N51 becomes(VDD−(VH−VL)) by capacitive coupling of the capacitor 301. Consequently,the transistor 501 is turned on.

By turning on the transistor 501, the power source potential VDD issupplied to the node N52, so that the potential of the node N52 becomesVDD. Consequently, the capacitor 302 holds the potential difference(VDD−VH) between the potential VH (the H signal) of the wiring 106 andthe potential VDD of the node N52. In addition, the transistor 502 isturned off.

By turning off the transistor 502, the node N51 becomes a floating stateto keep the potential (VDD−(VH−VL)).

Next, the second operation of the offset circuit shown in FIG. 5A isdescribed with reference to FIG. 5C.

As described above, (VDD−VH) is held in the capacitor 302 by the firstoperation. Then, when the potential of the wiring 106 is changed from VHto VL, the potential of the node N52 becomes (VDD−(VH−VL)) by capacitivecoupling of the capacitor 302. Consequently, the transistor 502 isturned on.

By turning on the transistor 502, the power source potential VDD issupplied to the node N51, so that the potential of the node N51 becomesVDD. Consequently, the capacitor 301 holds the potential difference(VDD−VH) between the potential VH (the H signal) of the wiring 105 andthe potential VDD of the node N51. In addition, the transistor 501 isturned off.

By turning off the transistor 501, the node N52 becomes the floatingstate to keep the potential (VDD−(VH−VL)).

By the first and second operations described above, the offset circuitshown in FIG. 5A operates as follows: In the first operation, the nodeN51 is brought into the floating state to keep the potential(VDD−(VH−VL)), and the power source potential VDD is supplied to thenode N52; and in the second operation, the power source potential VDD issupplied to the node N51, and the node N52 is brought into the floatingstate to keep the potential (VDD−(VH−VL)).

Therefore, as for a signal generated from the offset circuit shown inFIG. 5A, an H signal is VDD and an L signal is (VDD−(VH−VL)). That is,the circuit 101 shown in FIG. 5A can generate a signal based on thepower source potential VDD.

Note that, similarly to the offset circuit shown in FIG. 3A, althoughthe potential of the L signal of the signal generated from the offsetcircuit shown in FIG. 5A is (VDD−(VH−VL)), it is, in practice, a littlehigher than (VDD−(VH−VL)).

Further, similarly to the offset circuit shown in FIG. 3A, when the nodeN51 is connected to the wiring 107 shown in FIG. 1A, a signal of which Hand L levels are the same as those of the signal supplied to the wiring105 can be supplied to the wiring 107. Similarly, when the node N52 isconnected to the wiring 107 shown in FIG. 1A, a signal of which H and Llevels are reverse to those of the signal supplied to the wiring 105 canbe supplied to the wiring 107.

Further, similarly to the offset circuit shown in FIG. 3A, in the casewhere the node N51 is connected to the wiring 107, it is preferable thatthe capacitance of the capacitor 302 be smaller than that of thecapacitor 301.

Further, similarly to the offset circuit shown in FIG. 3A, in the casewhere the node N52 is connected to the wiring 107, it is preferable thatthe capacitance of the capacitor 301 be smaller than that of thecapacitor 302.

Further, similarly to each offset circuit shown in FIGS. 4A and 4B, aMOS capacitor may be employed as each of the capacitors 301 and 302. Asshown in FIG. 6A, a P-channel transistor 601 may be used instead of thecapacitor 301 and a P-channel transistor 602 may be used instead of thecapacitor 302. Note here that a gate of the transistor 601 is connectedto the wiring 105, and first and second terminals of the transistor 601are connected to the node N51. Similarly, a gate of the transistor 602is connected to the wiring 106, and first and second terminals of thetransistor 602 are connected to the node N52.

Further, similarly to each offset circuit shown in FIGS. 4A and 4B, asshown in FIG. 6B, N-channel transistors 603 and 604 can also be used asthe capacitors 301 and 302. Note here that a gate of the transistor 603is connected to the node N51, and first and second terminals of thetransistor 603 are connected to the wiring 105. Similarly, a gate of thetransistor 604 is connected to the node N52, and first and secondterminals of the transistor 604 are connected to the wiring 106.

Further, similarly to each offset circuit shown in FIGS. 4A and 4B, inFIG. 6A, in the case where the node N51 is connected to the wiring 107,it is preferable that the channel region of the transistor 602 besmaller than that of the transistor 601. In the case where the node N52is connected to the wiring 107, it is preferable that the channel regionof the transistor 602 be larger than that of the transistor 601.

Similarly, in FIG. 6B, in the case where the node N51 is connected tothe wiring 107, it is preferable that the channel region of thetransistor 604 be smaller than that of the transistor 603. In the casewhere the node N52 is connected to the wiring 107, it is preferable thatthe channel region of the transistor 604 be larger than that of thetransistor 603.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 3

This embodiment mode describes structure examples of the circuit 101(the offset circuit) included in the level shifter described inEmbodiment Mode 1. Note that in this embodiment mode, description ismade on the case where each signal supplied to the wirings 105 and 106is supplied to the wiring 108 after the potential is shifted toward an Lside while the timing is not changed (or is reversed) and the amplitudevoltage is not changed so much.

Note that in this embodiment mode and Embodiment Modes 1 and 2,reference numerals are used in common, and detailed description of thesame portions and portions having similar functions is not repeated.

First, a structure example of an offset circuit is described withreference to FIG. 7A.

An offset circuit shown in FIG. 7A includes a capacitor 701, a capacitor702, a transistor 703, and a transistor 704.

As shown in the offset circuit in FIG. 7A, a first electrode of thecapacitor 701 is connected to the wiring 105. A first electrode of thecapacitor 702 is connected to the wiring 106. A gate of the transistor703 is connected to a second electrode of the capacitor 701, a firstterminal of the transistor 703 is connected to a wiring 104, and asecond terminal of the transistor 703 is connected to a second electrodeof the capacitor 702. A gate of the transistor 704 is connected to thesecond electrode of the capacitor 702, a first terminal of thetransistor 704 is connected to the wiring 104, and a second terminal ofthe transistor 704 is connected to the second electrode of the capacitor701. Note that a connection point of the second electrode of thecapacitor 701, the gate of the transistor 703, and the second terminalof the transistor 704 is denoted by a node N71. A connection point ofthe second electrode of the capacitor 702, the second terminal of thetransistor 703, and the gate of the transistor 704 is denoted by a nodeN72. Either one of the node N71 or the node N72 is connected to thewiring 108 shown in FIG. 1A.

Next, operations of the offset circuit shown in FIG. 7A are describedwith reference to FIGS. 7B and 7C.

FIG. 7B shows an operation of the offset circuit shown in FIG. 7A when asignal of the wiring 105 is changed from an H signal to an L signal anda signal of the wiring 106 is changed from an L signal to an H signal.

FIG. 7C shows an operation of the offset circuit shown in FIG. 7A whenthe signal of the wiring 105 is changed from the L signal to the Hsignal and the signal of the wiring 106 is changed from the H signal tothe L signal. That is, the offset circuit shown in FIG. 7A repeats theoperation shown in FIG. 7B and the operation shown in FIG. 7C at acertain timing. Note here that the operation shown in FIG. 7B is denotedby a first operation and the operation shown in FIG. 7C is denoted by asecond operation.

Note that a value of (VH−VL) is equal to or larger than each thresholdvoltage of the transistors 703 and 704.

First, the first operation of the offset circuit shown in FIG. 7A isdescribed with reference to FIG. 7B. Note here that an initial potentialof the node N72 is VSS.

In an initial state, the capacitor 702 holds a potential difference(VL−VSS) between a potential VL (the L signal) of the wiring 106 and thepotential VSS of the node N72. Then, when the potential of the wiring106 is changed from VL to VH, the potential of the node N72 becomes(VSS+(VH−VL)) by capacitive coupling of the capacitor 702. Consequently,the transistor 704 is turned on.

By turning on the transistor 704, the power source potential VSS issupplied to the node N71, so that the potential of the node N71 becomesVSS. Consequently, the capacitor 701 holds the potential difference(VL−VSS) between the potential VL (the L signal) of the wiring 105 andthe potential VSS of the node N71. In addition, the transistor 703 isturned off.

By turning off the transistor 703, the node N72 becomes a floating stateto keep the potential (VSS+(VH−VL)).

Next, the second operation of the offset circuit shown in FIG. 7A isdescribed with reference to FIG. 7C.

As described above, (VL−VSS) is held in the capacitor 701 by the firstoperation. Then, when the potential of the wiring 105 is changed from VLto VH, the potential of the node N71 becomes (VSS+(VH−VL)) by capacitivecoupling of the capacitor 701. Consequently, the transistor 703 isturned on.

By turning on the transistor 703, the power source potential VSS issupplied to the node N72, so that the potential of the node N72 becomesVSS. Consequently, the capacitor 702 holds the potential difference(VL−VSS) between the potential VL (the L signal) of the wiring 106 andthe potential VSS of the node N72. In addition, the transistor 704 isturned off.

By turning off the transistor 704, the node N71 becomes the floatingstate to keep the potential (VSS+(VH−VL)).

Here, functions of the capacitors 701 and 702 and the transistors 703and 704 are described respectively.

First, the capacitor 701 holds the potential difference between thepotential VL of the wiring 105 and the potential VSS of the node N71 inthe first operation. Then, in the second operation, the capacitor 701increases the potential of the node N71 in accordance with increase ofpotential of the wiring 105 by capacitive coupling.

The capacitor 702 holds the potential difference between the potentialVL of the wiring 106 and the potential VSS of the node N72 in the secondoperation. Then, in the first operation, the capacitor 702 increases thepotential of the node N72 in accordance with increase of potential ofthe wiring 106 by capacitive coupling.

The transistor 703 functions as a switch of selecting whether the wiring104 and the node N72 are connected or not depending on the potential ofthe node N71. In addition, in the first operation, the transistor 703 isturned off so that the node N72 becomes the floating state. In thesecond operation, the transistor 703 is turned on so that the powersource potential VSS is supplied to the node N72.

The transistor 704 functions as a switch of selecting whether the wiring104 and the node N71 are connected or not depending on the potential ofthe node N72. In addition, in the first operation, the transistor 704 isturned on so that the power source potential VSS is supplied to the nodeN71. In the second operation, the transistor 704 is turned off so thatthe node N71 becomes the floating state.

By the first and second operations described above, the offset circuitshown in FIG. 7A operates as follows: In the first operation, the powersource potential VSS is supplied to the node N71, and the node N72 isbrought into the floating state to keep the potential (VSS+(VH−VL)); andin the second operation, the node N71 is brought into the floating stateto keep the potential (VSS+(VH−VL)), and the power source potential VSSis supplied to the node N72.

Therefore, as for a signal generated from the offset circuit shown inFIG. 7A, an H signal is (VSS+(VH−VL)) and an L signal is VSS. That is,the offset circuit shown in FIG. 7A can generate a signal based on thepower source potential VSS.

Note that as each of the potential of the node N72 in the firstoperation and the potential of the node N71 in the second operation,(VSS+(VH−VL)) is kept. In practice, however, each of the potential ofthe node N72 in the first operation and the potential of the node N71 inthe second operation becomes lower than (VSS+(VH−VL)) due to wiringcapacitance, parasitic capacitance, or the like. Therefore, in order toreduce such an effect of the wiring capacitance or the parasiticcapacitance, each capacitance of the capacitors 701 and 702 may be setto be sufficiently larger than the wiring capacitance or the parasiticcapacitance.

When the node N71 is connected to the wiring 108 shown in FIG. 1A, asignal of which H and L levels are the same as those of the signalsupplied to the wiring 105 can be supplied to the wiring 108. Similarly,when the node N72 is connected to the wiring 108 shown in FIG. 1A, asignal of which H and L levels are reverse to those of the signalsupplied to the wiring 105 can be supplied to the wiring 108.

In the case where the node N71 is connected to the wiring 108, it ispreferable that the capacitance of the capacitor 702 be smaller thanthat of the capacitor 701. This is because, as described above, since itis described above that the capacitance of the capacitor 701 may be setto be sufficiently larger than the wiring capacitance or the parasiticcapacitance, the potential of the node N72 is not necessarily(VSS+(VH−VL)) as long as the transistor 704 can be turned on. Thus thecapacitance of the capacitor 702 can be smaller than that of thecapacitor 701, thereby an element region of the capacitor 702 can bereduced.

In the case where the node N72 is connected to the wiring 108, it ispreferable that the capacitance of the capacitor 701 be smaller thanthat of the capacitor 702 because of the similar reason to the casewhere the node N71 is connected to the wiring 108.

Note that each of the capacitors 701 and 702 may employ a structure inwhich an insulating layer is interposed between two electrode layers. Byemploying the structure in which an insulating layer is interposedbetween two electrode layers for each of the capacitors 701 and 702, thecapacitors 701 and 702 can keep the capacitance constant regardless ofan applied voltage, so that the level shifter of the present inventioncan be stably operated.

Further, it is preferable that an insulating layer of each of thecapacitors 701 and 702 be a gate insulating film. This is because thecapacitors 701 and 702 can obtain capacitance effectively since a gateinsulating film is generally thinner than other insulating films (e.g.,an interlayer film or a planarizing film).

Note also that a MOS capacitor may be employed as each of the capacitors701 and 702. FIG. 8A shows a structure in the case where MOS capacitorsare employed as the capacitors 701 and 702. In an offset circuit shownin FIG. 8A, an N-channel transistor 801 is used instead of the capacitor701 and an N-channel transistor 802 is used instead of the capacitor702. Note here that a gate of the transistor 801 is connected to thewiring 105, and first and second terminals of the transistor 801 areconnected to the node N71. This is because the transistor 801 canoperate as a capacitor since the potential of the wiring 105 is higherthan that of the node N71 to turn on the transistor 801 so that achannel is formed in a channel region of the transistor 901. Similarly,a gate of the transistor 802 is connected to the wiring 106, and firstand second terminals of the transistor 802 are connected to the nodeN72. This is because the transistor 802 can operate as a capacitor sincethe potential of the wiring 106 is higher than that of the node N72 toturn on the transistor 802 so that a channel is formed in a channelregion of the transistor 802.

Further, as shown in FIG. 6B, a P-channel transistor may also be used aseach capacitor. In an offset circuit shown in FIG. 6B, a P-channeltransistor 803 is used instead of the capacitor 701 and a P-channeltransistor 804 is used instead of the capacitor 702. Note here that agate of the transistor 803 is connected to the node N71, and first andsecond terminals of the transistor 803 are connected to the wiring 105.This is because the transistor 803 can operate as a capacitor since thepotential of the node N71 is lower than that of the wiring 105 to turnon the transistor 803 so that a channel is formed in a channel region ofthe transistor 803. Similarly, a gate of the transistor 804 is connectedto the node N72, and first and second terminals of the transistor 804are connected to the wiring 106. This is because the transistor 804 canoperate as a capacitor since the potential of the node N72 is lower thanthat of the wiring 106 to turn on the transistor 804 so that a channelis formed in a channel region of the transistor 804.

Note that as described above, in FIG. 8A, in the case where the node N71is connected to the wiring 108, it is preferable that the channel region(L×W: L is channel length and W is channel width) of the transistor 802be smaller than that of the transistor 801. In the case where the nodeN72 is connected to the wiring 108, it is preferable that the channelregion of the transistor 802 be larger than that of the transistor 801.

Similarly, in FIG. 8B, in the case where the node N71 is connected tothe wiring 108, it is preferable that the channel region (L×W: L ischannel length and W is channel width) of the transistor 804 be smallerthan that of the transistor 803. In the case where the node N72 isconnected to the wiring 108, it is preferable that the channel region ofthe transistor 804 be larger than that of the transistor 803.

Here, although the offset circuit shown in FIG. 7A includes theN-channel transistors and the capacitors, it may include P-channeltransistors and capacitors. FIG. 9A shows an offset circuit includingP-channel transistors and capacitors.

An offset circuit shown in FIG. 9A includes the capacitor 701, thecapacitor 702, a transistor 901, and a transistor 902.

Note that the transistors 901 and 902 correspond to and have similarfunctions to the transistors 703 and 704 in FIG. 7A. Nodes N91 and N92correspond to the nodes N71 and N72 in FIG. 7A.

As shown in the offset circuit in FIG. 9A, a first electrode of thecapacitor 701 is connected to the wiring 105. A first electrode of thecapacitor 702 is connected to the wiring 106. A gate of the transistor901 is connected to a second electrode of the capacitor 701, a firstterminal of the transistor 901 is connected to the wiring 104, and asecond terminal of the transistor 901 is connected to a second electrodeof the capacitor 702. A gate of the transistor 902 is connected to thesecond electrode of the capacitor 702, a first terminal of thetransistor 902 is connected to the wiring 104, and a second terminal ofthe transistor 902 is connected to the second electrode of the capacitor701. Note that a connection point of the second electrode of thecapacitor 701, the gate of the transistor 901, and the second terminalof the transistor 902 is denoted by the node N91. A connection point ofthe second electrode of the capacitor 702, the second terminal of thetransistor 901, and the gate of the transistor 902 is denoted by thenode N92. Either one of the node N91 or the node N92 is connected to thewiring 108 shown in FIG. 1A.

Next, operations of the offset circuit shown in FIG. 9A are describedwith reference to FIGS. 9B and 9C.

FIG. 9B shows an operation of the offset circuit shown in FIG. 9A when asignal of the wiring 105 is changed from an H signal to an L signal anda signal of the wiring 106 is changed from an L signal to an H signal.

FIG. 9C shows an operation of the offset circuit shown in FIG. 9A whenthe signal of the wiring 105 is changed from the L signal to the Hsignal and the signal of the wiring 106 is changed from the H signal tothe L signal. That is, the offset circuit shown in FIG. 9A repeats theoperation shown in FIG. 9B and the operation shown in FIG. 9C at acertain timing. Note here that the operation shown in FIG. 9B is denotedby a first operation and the operation shown in FIG. 9C is denoted by asecond operation.

First, the first operation of the offset circuit shown in FIG. 9A isdescribed with reference to FIG. 9B. Note here that an initial potentialof the node N91 is VSS.

In an initial state, the capacitor 701 holds a potential difference(VH−VSS) between a potential VH (the H signal) of the wiring 105 and thepotential VSS of the node N91. Then, when the potential of the wiring105 is changed from VH to VL, the potential of the node N91 becomes(VSS−(VH−VL)) by capacitive coupling of the capacitor 701. Consequently,the transistor 901 is turned on.

By turning on the transistor 901, the power source potential VSS issupplied to the node N92, so that the potential of the node N92 becomesVSS. Consequently, the capacitor 702 holds the potential difference(VH−VSS) between the potential VH (the H signal) of the wiring 106 andthe potential VSS of the node N92. In addition, the transistor 902 isturned off.

By turning off the transistor 902, the node N91 becomes a floating stateto keep the potential (VSS−(VH−VL)).

Next, the second operation of the offset circuit shown in FIG. 9A isdescribed with reference to FIG. 9C.

As described above, (VH−VSS) is held in the capacitor 702 by the firstoperation. Then, when the potential of the wiring 106 is changed from VHto VL, the potential of the node N92 becomes (VSS−(VH−VL)) by capacitivecoupling of the capacitor 702. Consequently, the transistor 902 isturned on.

By turning on the transistor 902, the power source potential VSS issupplied to the node N91, so that the potential of the node N91 becomesVSS. Consequently, the capacitor 701 holds the potential difference(VH−VSS) between the potential VH (the H signal) of the wiring 105 andthe potential VSS of the node N91. In addition, the transistor 901 isturned off.

By turning off the transistor 901, the node N92 becomes the floatingstate to keep the potential (VSS−(VH−VL)).

By the first and second operations described above, the offset circuitshown in FIG. 9A operates as follows: In the first operation, the nodeN91 is brought into the floating state to keep the potential(VSS−(VH−VL)), and the power source potential VSS is supplied to thenode N92; and in the second operation, the power source potential VSS issupplied to the node N91, and the node N92 is brought into the floatingstate to keep the potential (VSS−(VH−VL)).

Therefore, as for a signal generated from the offset circuit shown inFIG. 9A, an H signal is VSS and an L signal is (VSS−(VH−VL)). That is,the offset circuit shown in FIG. 9A can generate a signal based on thepower source potential VSS.

Note that, similarly to the offset circuit shown in FIG. 7A, althoughthe potential of the L signal of the signal generated from the offsetcircuit shown in FIG. 9A is (VSS−(VH−VL)), it is, in practice, a littlehigher than (VSS−(VH−VL)).

Further, similarly to the offset circuit shown in FIG. 7A, when the nodeN91 is connected to the wiring 108 shown in FIG. 1A, a signal of which Hand L levels are the same as those of the signal supplied to the wiring105 can be supplied to the wiring 108. Similarly, when the node N92 isconnected to the wiring 108 shown in FIG. 1A, a signal of which H and Llevels are reverse to those of the signal supplied to the wiring 105 canbe supplied to the wiring 108.

Further, similarly to the offset circuit shown in FIG. 7A, in the casewhere the node N91 is connected to the wiring 108, it is preferable thatthe capacitance of the capacitor 702 be smaller than that of thecapacitor 701.

Further, similarly to the offset circuit shown in FIG. 7A, in the casewhere the node N92 is connected to the wiring 108, it is preferable thatthe capacitance of the capacitor 701 be smaller than that of thecapacitor 702.

Further, similarly to each offset circuit shown in FIGS. 8A and 8B, aMOS capacitor may be employed as each of the capacitors 701 and 702. Asshown in FIG. 10A, a P-channel transistor 1091 may be used instead ofthe capacitor 701 and a P-channel transistor 1092 may be used instead ofthe capacitor 702. Note here that a gate of the transistor 1091 isconnected to the node N91, and first and second terminals of thetransistor 1091 are connected to the wiring 105. Similarly, a gate ofthe transistor 1092 is connected to the node N92, and first and secondterminals of the transistor 1092 are connected to the wiring 106.

Further, similarly to each offset circuit shown in FIGS. 8A and 8B, asshown in FIG. 10B, N-channel transistors 1093 and 1094 can also be usedas the capacitors 701 and 702. Note here that a gate of the transistor1093 is connected to the wiring 105, and first and second terminals ofthe transistor 1093 are connected to the node N91. Similarly, a gate ofthe transistor 1094 is connected to the wiring 106, and first and secondterminals of the transistor 1094 are connected to the node N92.

Further, similarly to each offset circuit shown in FIGS. 8A and 8B, inFIG. 10A, in the case where the node N91 is connected to the wiring 108,it is preferable that the channel region of the transistor 1092 besmaller than that of the transistor 1091. In the case where the node N92is connected to the wiring 108, it is preferable that the channel regionof the transistor 1092 be larger than that of the transistor 1091.

Similarly, in FIG. 10B, in the case where the node N91 is connected tothe wiring 108, it is preferable that the channel region of thetransistor 1094 be smaller than that of the transistor 1093. In the casewhere the node N92 is connected to the wiring 108, it is preferable thatthe channel region of the transistor 1094 be larger than that of thetransistor 1093.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 4

This embodiment mode describes structure examples of the circuit 101(the offset circuit) included in the level shifter described inEmbodiment Mode 1. Note that in this embodiment mode, description ismade on the case where each signal supplied to the wirings 105 and 106is supplied to the wirings 107 and 108 after each potential is shiftedtoward an H side and an L side while the timing is not changed (or isreversed) and the amplitude voltage is not changed so much.

Note that in this embodiment mode and Embodiment Modes 1 to 3, referencenumerals are used in common, and detailed description of the sameportions and portions having similar functions is not repeated.

First, a structure example of an offset circuit is described withreference to FIG. 11.

An offset circuit shown in FIG. 11 includes the capacitor 301, thecapacitor 302, the transistor 303, the transistor 304, the capacitor701, the capacitor 702, the transistor 703, and the transistor 704.

As shown in the offset circuit in FIG. 11, a first electrode of thecapacitor 301 is connected to the wiring 105. A first electrode of thecapacitor 302 is connected to the wiring 106. A gate of the transistor303 is connected to a second electrode of the capacitor 301, a firstterminal of the transistor 303 is connected to the wiring 103, and asecond terminal of the transistor 303 is connected to a second electrodeof the capacitor 302. A gate of the transistor 304 is connected to thesecond electrode of the capacitor 302, a first terminal of thetransistor 304 is connected to the wiring 103, and a second terminal ofthe transistor 304 is connected to the second electrode of the capacitor301. A first electrode of the capacitor 702 is connected to the wiring106. A gate of the transistor 703 is connected to a second electrode ofthe capacitor 701, a first terminal of the transistor 703 is connectedto the wiring 104, and a second terminal of the transistor 703 isconnected to a second electrode of the capacitor 702. A gate of thetransistor 704 is connected to the second electrode of the capacitor702, a first terminal of the transistor 704 is connected to the wiring104, and a second terminal of the transistor 704 is connected to thesecond electrode of the capacitor 701.

Note that the connection point of the second electrode of the capacitor301, the gate of the transistor 303, and the second terminal of thetransistor 304 is denoted by the node N31. The connection point of thesecond electrode of the capacitor 302, the second terminal of thetransistor 303, and the gate of the transistor 304 is denoted by thenode N32. The connection point of the second electrode of the capacitor701, the gate of the transistor 703, and the second terminal of thetransistor 704 is denoted by the node N71. The connection point of thesecond electrode of the capacitor 702, the second terminal of thetransistor 703, and the gate of the transistor 704 is denoted by thenode N72.

Either one of the node N31 or the node N32 is connected to the wiring107 shown in FIG. 1A. Either one of the node N71 or the node N72 isconnected to the wiring 108 shown in FIG. 1A.

The offset circuit shown in FIG. 3A includes the capacitor 301, thecapacitor 302, the transistor 303, and the transistor 304. The offsetcircuit shown in FIG. 7A includes the capacitor 701, the capacitor 702,the transistor 703, and the transistor 704.

As described above, the power source potential VDD and the power sourcepotential VSS are supplied to the wirings 103 and 104. As for the signalof the wiring 105, H and L levels are reverse to those of the signal ofthe wiring 106.

Operations of the offset circuit shown in FIG. 11 are similar to thosein FIGS. 3A and 7A, thus description thereof is omitted.

Note also that as described above, a MOS capacitor may be employed aseach of the capacitors 301, 302, 701, and 702. FIG. 12 shows a structurein the case where MOS capacitors are employed as the capacitors 301,302, 701, and 702.

As shown in an offset circuit in FIG. 12, the transistors 401, 402, 801,and 802 can be used instead of the capacitors 301, 302, 701, and 702,respectively. Note that the transistors 401, 402, 801, and 802 areN-channel transistors.

Note here that a gate of the transistor 401 is connected to the nodeN31, and first and second terminals of the transistor 401 are connectedto the wiring 105. A gate of the transistor 402 is connected to the nodeN32, and first and second terminals of the transistor 402 are connectedto the wiring 106. A gate of the transistor 801 is connected to thewiring 105, and first and second terminals of the transistor 801 areconnected to the node N71. A gate of the transistor 802 is connected tothe wiring 106, and first and second terminals of the transistor 802 areconnected to the node N72.

Here, although the offset circuit shown in FIG. 11 includes theN-channel transistors and the capacitors, it may include P-channeltransistors and capacitors. FIG. 13 shows an offset circuit includingP-channel transistors and capacitors.

An offset circuit shown in FIG. 13 includes the capacitor 301, thecapacitor 302, the transistor 501, the transistor 502, the capacitor701, the capacitor 702, the transistor 901, and the transistor 902.

As shown in the offset circuit in FIG. 13, a first electrode of thecapacitor 301 is connected to the wiring 105. A first electrode of thecapacitor 302 is connected to the wiring 106. A gate of the transistor501 is connected to a second electrode of the capacitor 301, a firstterminal of the transistor 501 is connected to the wiring 103, and asecond terminal of the transistor 501 is connected to a second electrodeof the capacitor 302. A gate of the transistor 502 is connected to thesecond electrode of the capacitor 302, a first terminal of thetransistor 502 is connected to the wiring 103, and a second terminal ofthe transistor 502 is connected to the second electrode of the capacitor301. A first electrode of the capacitor 702 is connected to the wiring106. A gate of the transistor 901 is connected to a second electrode ofthe capacitor 701, a first terminal of the transistor 901 is connectedto the wiring 104, and a second terminal of the transistor 901 isconnected to the second electrode of the capacitor 702. A gate of thetransistor 902 is connected to a second electrode of the capacitor 702,a first terminal of the transistor 902 is connected to the wiring 104,and a second terminal of the transistor 902 is connected to the secondelectrode of the capacitor 701.

Note that the connection point of the second electrode of the capacitor301, the gate of the transistor 501, and the second terminal of thetransistor 502 is denoted by the node N51. The connection point of thesecond electrode of the capacitor 302, the second terminal of thetransistor 501, and the gate of the transistor 502 is denoted by thenode N52. The connection point of the second electrode of the capacitor701, the gate of the transistor 901, and the second terminal of thetransistor 902 is denoted by the node N91. The connection point of thesecond electrode of the capacitor 702, the second terminal of thetransistor 901, and the gate of the transistor 902 is denoted by thenode N92.

Either one of the node N51 or the node N52 is connected to the wiring107 shown in FIG. 1A. Either one of the node N91 or the node N92 isconnected to the wiring 108 shown in FIG. 1A.

The offset circuit shown in FIG. 5A includes the capacitor 301, thecapacitor 302, the transistor 503, and the transistor 504. The offsetcircuit shown in FIG. 9A includes the capacitor 701, the capacitor 702,the transistor 903, and the transistor 904.

As described above, the power source potential VDD and the power sourcepotential VSS are supplied to the wirings 103 and 104. As for the signalof the wiring 105, H and L levels are reverse to those of the signal ofthe wiring 106.

Operations of the offset circuit shown in FIG. 13 are similar to thosein FIGS. 5A and 9A, thus description thereof is omitted.

Note also that as described above, a MOS capacitor may be employed aseach of the capacitors 301, 302, 701, and 702. FIG. 14 shows a structurein the case where MOS capacitors are employed as the capacitors 301,302, 701, and 702.

As shown in an offset circuit shown in FIG. 14, the transistors 601,602, 1091, and 1092 can be used instead of the capacitors 301, 302, 701,and 702, respectively. Note that the transistors 601, 602, 1091, and1092 are P-channel transistors.

Note here that a gate of the transistor 601 is connected to the wiring105, and first and second terminals of the transistor 601 are connectedto the node N51. A gate of the transistor 602 is connected to the wiring106, and first and second terminals of the transistor 602 are connectedto the node N52. A gate of the transistor 1091 is connected to the nodeN91, and first and second terminals of the transistor 1091 are connectedto the wiring 105. A gate of the transistor 1092 is connected to thenode N92, and first and second terminals of the transistor 1092 areconnected to the wiring 106.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 5

This embodiment mode describes specific structures of the level shifterdescribed in Embodiment Mode 1.

Note that in this embodiment mode and Embodiment Modes 1 to 4, referencenumerals are used in common, and detailed description of the sameportions and portions having similar functions is not repeated.

First, a specific structure example of a level shifter of the presentinvention is described with reference to FIG. 15.

A level shifter shown in FIG. 15 includes the capacitor 701, thecapacitor 702, the transistor 703, the transistor 704, a transistor1501, and a transistor 1502.

As shown in the level shifter in FIG. 15, a first electrode of thecapacitor 701 is connected to the wiring 105. A first electrode of thecapacitor 702 is connected to the wiring 106. A gate of the transistor703 is connected to a second electrode of the capacitor 701, a firstterminal of the transistor 703 is connected to the wiring 104, and asecond terminal of the transistor 703 is connected to a second electrodeof the capacitor 702. A gate of the transistor 704 is connected to asecond electrode of the capacitor 702, a first terminal of thetransistor 704 is connected to the wiring 104, and a second terminal ofthe transistor 704 is connected to the second electrode of the capacitor701. Note that a connection point of the second electrode of thecapacitor 701, the gate of the transistor 703, and the second terminalof the transistor 704 is denoted by the node N71. A connection point ofthe second electrode of the capacitor 702, the second terminal of thetransistor 703, and the gate of the transistor 704 is denoted by thenode N72. A gate of the transistor 1502 is connected to the node N72, afirst terminal of the transistor 1502 is connected to the wiring 104,and a second terminal of the transistor 1502 is connected to the wiring109. A gate of the transistor 1501 is connected to the wiring 103, afirst terminal of the transistor 1501 is connected to the wiring 103,and a second terminal of the transistor 1502 is connected to the wiring109.

An offset circuit 1503 includes the capacitor 701, the capacitor 702,the transistor 703, and the transistor 704. The offset circuit 1503 issimilar to the offset circuit shown in FIG. 7A.

A logic circuit 1500 includes the transistor 1501 and the transistor1502. The logic circuit 1500 corresponds to the circuit 102 shown inFIGS. 1A to 1C.

Note that the transistors 1501 and 1502 are N-channel transistors.Therefore, since the level shifter shown in FIG. 15 can be formed byusing the transistors of only the N-channel type, amorphous silicon canbe used for a semiconductor layer in the level shifter shown in FIG. 15so that a manufacturing process can be simplified. Consequently,manufacturing cost can be reduced and a yield can be improved. Further,a large semiconductor device can also be manufactured.

Further, in the level shifter shown in FIG. 15, the manufacturingprocess can be simplified even if polysilicon or single crystallinesilicon is used for the semiconductor layer.

Next, operations of the level shifter shown in FIG. 15 are describedwith reference to timing charts of FIGS. 16A to 16C. However, timing ofpotential change in the timing charts shown in FIGS. 16A to 16C can bedetermined as appropriate, and is not limited to that in the timingcharts of FIGS. 16A to 16C.

In the timing chart shown in FIG. 16A, a signal (a potential) suppliedto the wiring 105 and a potential of the node N71 are shown. In thetiming chart shown in FIG. 16B, a signal (a potential) supplied to thewiring 106 and a potential of the node N72 are shown. In the timingchart shown in FIG. 16C, a signal (a potential) supplied to the wiring109 is shown.

FIG. 17 shows an operation of the level shifter shown in FIG. 15 when anL signal is supplied to the wiring 105 and an H signal is supplied tothe wiring 106. FIG. 18 shows an operation of the level shifter shown inFIG. 15 when an H signal is supplied to the wiring 105 and an L signalis supplied to the wiring 106. Note here that the operation shown inFIG. 17 is denoted by a first operation and the operation shown in FIG.18 is denoted by a second operation.

Since operations of the offset circuit 1503 are similar to those of theoffset circuit shown in FIG. 7A, specific description thereof isomitted.

First, the first operation of the level shifter shown in FIG. 15 isdescribed with reference to the timing charts shown in FIGS. 16A to 16Cand FIG. 17.

When the wiring 105 becomes an L level, the potential of the node N71becomes VSS. On the other hand, when the wiring 106 becomes an H level,the potential of the node N72 becomes (VSS+(VH−VL)). Consequently, thetransistor 1502 is turned on, and the power source potential VSS issupplied to the wiring 109, so that an L signal is outputted from thewiring 109. Note that the potential of the wiring 109 is determined byoperating points of the transistors 1501 and 1502, thus it is a littlehigher than the power source potential VSS.

Next, the second operation of the level shifter shown in FIG. 15 isdescribed with reference to the timing charts shown in FIGS. 16A to 16Cand FIG. 18.

When the wiring 105 becomes an H level, the potential of the node N71becomes (VSS+(VH−VL)). On the other hand, when the wiring 106 becomes anL level, the potential of the node N72 becomes VSS. Consequently, thetransistor 1502 is turned off, and the power source potential VDD issupplied to the wiring 109, so that the potential of the wiring 109 isincreased. This potential increase of the wiring 109 continues until thepotential of the wiring 109 becomes a potential (VDD−Vth1501) obtainedby subtracting a threshold voltage Vth1501 of the transistor 1501 fromthe power source potential VDD and the transistor 1501 is turned off.Accordingly, the potential of the wiring 109 becomes (VDD−Vth1501), andan H signal is outputted from the wiring 109.

Here, functions of the logic circuit 1500 and the offset circuit 1503are described.

First, the offset circuit 1503 has a similar function to the offsetcircuit shown in FIG. 7A. Further, the offset circuit 1503 generates anoffset signal of which H-signal and L-signal potentials are(VSS+(VH−VL)) and VSS from a control signal of which H-signal andL-signal potentials are VH and VL, and supplies the offset signal to thelogic circuit 1500.

The logic circuit 1500 generates an output signal of which H-signal andL-signal potentials are (VDD−Vth1501) and approximately VSS,respectively, from the offset signal of which H-signal and L-signalpotentials are (VSS+(VH−VL)) and VSS, respectively, and supplies theoutput signal to the wiring 109.

Here, functions of the transistors 1501 and 1502 are described.

First, the transistor 1501 has a function as a diode. Input terminalsthereof are the gate and the first terminal, and an output terminalthereof is the second terminal. The transistor 1501 is any elementhaving a resistance component. By using a resistor instead of thetransistor 1501, the potential of the wiring 109 can be made equal tothe power source potential VDD in the second operation in the logiccircuit 1500.

The transistor 1502 has a function as a switch of selecting whether thewiring 104 and the wiring 109 are connected to each other or notdepending on the potential of the node N72. The transistor 1502 isturned on and supplies the power source potential VSS to the wiring 109in the first operation.

By the above-described first and second operations, the level shiftershown in FIG. 15 can shift each control signal supplied to the wirings105 and 106 such that the H-signal potential is shifted from VH to VDDand the L-signal potential is shifted from VL to VSS, and then outputthrough the wiring 109.

Since the gate voltage of the transistor 1502 is (VSS+(VH−VL)) in thefirst operation and is VSS in the second operation, through current ofthe logic circuit 1500 is reduced. This is because the amplitude voltageof the gate of the transistor 1502 is small (VH−VL). Therefore, powerconsumption of a semiconductor device including the level shifter shownin FIG. 15 is reduced because through current of the logic circuit 1500is small.

Further, since the amplitude voltage of the gate of the transistor 1502is small, noise generated in the logic circuit 1500 is reduced. This isbecause noise generated through parasitic capacitance between the gateand the second terminal (the wiring 109) of the transistor 1502 becomessmall.

Note also that as described above, a MOS capacitor may be employed aseach of the capacitors 701 and 702. Further, in the case of the levelshifter shown in FIG. 15, it is preferable to use an N-channeltransistor for each capacitor similarly to the offset circuit shown inFIG. 8A. This is because by forming each capacitor using an N-channeltransistor, amorphous silicon can be used for a semiconductor layer inthe level shifter shown in FIG. 15 so that a manufacturing process canbe simplified. Consequently, manufacturing cost can be reduced and ayield can be improved. Further, a large semiconductor device can also bemanufactured.

Further, in the level shifter shown in FIG. 15, the manufacturingprocess can be simplified even if polysilicon or single crystallinesilicon is used for the semiconductor layer.

Note that as shown in FIG. 19, the gate of the transistor 1502 may alsobe connected to the node N71 in the level shifter shown in FIG. 15. Inthe case where the gate of the transistor 1502 is connected to the nodeN71 (FIG. 19), H and L levels of the signal (the potential) of thewiring 109 are reverse to those in the case where the gate of thetransistor 1502 is connected to the node N72 (FIG. 15) as shown intiming charts in FIGS. 20A to 20C. Operations of a level shifter shownin FIG. 19 are similar to those of the level shifter shown in FIG. 15.Therefore, whether the gate of the transistor 1502 is connected to thenode N71 or the node N72 can be determined as appropriate.

Here, although the level shifter shown in FIG. 15 includes the N-channeltransistors and the capacitors, it may include P-channel transistors andcapacitors. FIG. 29 shows a level shifter including P-channeltransistors and capacitors.

A level shifter shown in FIG. 29 includes the capacitor 301, thecapacitor 302, the transistor 501, the transistor 502, a transistor2901, and a transistor 2902.

Note that the capacitor 301, the capacitor 302, the transistor 501, thetransistor 502, the transistor 2901, and the transistor 2902 correspondto and have similar functions to the capacitor 701, the capacitor 702,the transistor 703, the transistor 704, the transistor 1501, and thetransistor 1502 in FIG. 15 respectively. A logic circuit 2900 and anoffset circuit 2903 correspond to and have similar functions to thelogic circuit 1500 and the offset circuit 1503 in FIG. 15 respectively.The nodes N51 and N52 correspond to the nodes N71 and N72 in FIG. 15respectively.

As shown in the level shifter in FIG. 29, a first electrode of thecapacitor 301 is connected to the wiring 105. A first electrode of thecapacitor 302 is connected to the wiring 106. A gate of the transistor501 is connected to a second electrode of the capacitor 301, a firstterminal of the transistor 501 is connected to the wiring 103, and asecond terminal of the transistor 501 is connected to a second electrodeof the capacitor 302. A gate of the transistor 502 is connected to thesecond electrode of the capacitor 302, a first terminal of thetransistor 502 is connected to the wiring 103, and a second terminal ofthe transistor 502 is connected to the second electrode of the capacitor301. Note that a connection point of the second electrode of thecapacitor 301, the gate of the transistor 501, and the second terminalof the transistor 502 is denoted by the node N51. A connection point ofthe second electrode of the capacitor 302, the second terminal of thetransistor 501, and the gate of the transistor 502 is denoted by thenode N52. A gate of the transistor 2902 is connected to the node N52, afirst terminal of the transistor 2902 is connected to the wiring 103,and a second terminal of the transistor 2902 is connected to the wiring109. A gate of the transistor 2901 is connected to the wiring 104, afirst terminal of the transistor 2901 is connected to the wiring 104,and a second terminal of the transistor 2901 is connected to the wiring109.

Next, operations of the level shifter shown in FIG. 29 are describedwith reference to timing charts of FIGS. 30A to 30C. However, timing ofpotential change in the timing charts shown in FIGS. 30A to 30C can bedetermined as appropriate, and is not limited to that in the timingcharts of FIGS. 30A to 30C.

In the timing chart shown in FIG. 30A, a signal (a potential) suppliedto the wiring 105 and a potential of the node N51 are shown. In thetiming chart shown in FIG. 30B, a signal (a potential) supplied to thewiring 106 and a potential of the node N52 are shown. In the timingchart shown in FIG. 30C, a signal (a potential) supplied to the wiring109 is shown.

FIG. 31 shows an operation of the level shifter shown in FIG. 29 when anL signal is supplied to the wiring 105 and an H signal is supplied tothe wiring 106. FIG. 32 shows an operation of the level shifter shown inFIG. 29 when an H signal is supplied to the wiring 105 and an L signalis supplied to the wiring 106. Note here that the operation shown inFIG. 31 is denoted by a first operation and the operation shown in FIG.32 is denoted by a second operation.

Since operations of the offset circuit 2903 are similar to those of theoffset circuit shown in FIG. 5A, specific description thereof isomitted.

First, the first operation of the level shifter shown in FIG. 29 isdescribed with reference to the timing charts shown in FIGS. 30A to 30Cand FIG. 31.

When the wiring 105 becomes an L level, the potential of the node N51becomes (VDD−(VH−VL)). On the other hand, when the wiring 106 becomes anH level, the potential of the node N52 becomes VDD. Consequently, thetransistor 2902 is turned off, and the power source potential VSS issupplied to the wiring 109, so that the potential of the wiring 109 isdecreased. This potential decrease of the wiring 109 continues until thepotential of the wiring 109 becomes a potential (VSS+|Vth2901|) obtainedby adding an absolute value of a threshold voltage Vth2901 of thetransistor 2901 to the power source potential VSS, and then, thetransistor 2901 is turned off. Accordingly, the potential of the wiring109 becomes (VSS+|Vth2901|), and an L signal is outputted from thewiring 109.

Next, the second operation of the level shifter shown in FIG. 29 isdescribed with reference to the timing charts shown in FIGS. 30A to 30Cand FIG. 32.

When the wiring 105 becomes an H level, the potential of the node N51becomes VDD. On the other hand, when the wiring 106 becomes an L level,the potential of the node N52 becomes (VDD−(VH−VL)). Consequently, thetransistor 2902 is turned on, and the power source potential VDD issupplied to the wiring 109, so that an H signal is outputted from thewiring 109. Note that the potential of the wiring 109 is determined byoperating points of the transistors 2901 and 2902, thus it is a littlelower than the power source potential VDD.

By the above-described first and second operations, the level shiftershown in FIG. 29 can shift each control signal supplied to the wirings105 and 106 such that the H-signal potential is shifted from VH to VDDand the L-signal potential is shifted from VL to VSS, and then outputthrough the wiring 109.

Since the gate voltage of the transistor 2902 is VDD in the firstoperation and is (VDD−(VH−VL)) in the second operation, through currentof the logic circuit 2900 is reduced. This is because the amplitudevoltage of the gate of the transistor 2902 is small (VH−VL). Therefore,power consumption of a semiconductor device including the level shiftershown in FIG. 29 is reduced because through current of the logic circuit2900 is small.

Further, since the amplitude voltage of the gate of the transistor 2902is small, noise generated in the logic circuit 2900 is reduced. This isbecause noise generated through parasitic capacitance between the gateand the second terminal (the wiring 109) of the transistor 2902 becomessmall.

Note also that as described above, a MOS capacitor may be employed aseach of the capacitors 301 and 302. Further, in the case of the levelshifter shown in FIG. 29, it is preferable to use a P-channel transistorfor each capacitor similarly to the offset circuit shown in FIG. 6A.

Note that as shown in FIG. 33, the gate of the transistor 2902 may alsobe connected to the node N51 in the level shifter shown in FIG. 29.Timing charts in the case where the gate of the transistor 2902 isconnected to the node N51 are shown in FIGS. 34A to 34C.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 6

This embodiment mode describes specific structures of the level shifterdescribed in Embodiment Mode 1, which are different from Embodiment Mode5.

Note that in this embodiment mode and Embodiment Modes 1 to 5, referencenumerals are used in common, and detailed description of the sameportions and portions having similar functions is not repeated.

First, a specific structure example of a level shifter of the presentinvention is described with reference to FIG. 21.

A level shifter shown in FIG. 21 includes the capacitor 701, thecapacitor 702, the transistor 703, the transistor 704, a transistor2101, a transistor 2102, a transistor 2103, and a transistor 2104.

As shown in the level shifter in FIG. 21, a gate of the transistor 2101is connected to the wiring 103, a first terminal of the transistor 2101is connected to the wiring 103, and a second terminal of the transistor2101 is connected to a wiring 109-2. A gate of the transistor 2102 isconnected to the node N72, a first terminal of the transistor 2102 isconnected to the wiring 104, and a second terminal of the transistor2102 is connected to the wiring 109-2. A gate of the transistor 2103 isconnected to the wiring 103, a first terminal of the transistor 2103 isconnected to the wiring 103, and a second terminal of the transistor2103 is connected to a wiring 109-1. A gate of the transistor 2104 isconnected to the node N71, a first terminal of the transistor 2104 isconnected to the wiring 104, and a second terminal of the transistor2104 is connected to the wiring 109-1.

The logic circuit 1500 shown in FIG. 15 includes the transistor 2101 andthe transistor 2102. Similarly, the logic circuit 1500 shown in FIG. 15includes the transistor 2103 and the transistor 2104. A logic circuit2100 includes the transistors 2101, 2102, 2103, and 2104.

Note that the transistors 2101, 2102, 2103, and 2104 are N-channeltransistors. Therefore, since the level shifter shown in FIG. 21 can beformed by using the transistors of only the N-channel type, amorphoussilicon can be used for a semiconductor layer in the level shifter shownin FIG. 21 so that a manufacturing process can be simplified.Consequently, manufacturing cost can be reduced and a yield can beimproved. Further, a large semiconductor device can also bemanufactured.

Further, in the level shifter shown in FIG. 21, the manufacturingprocess can be simplified even if polysilicon or single crystallinesilicon is used for the semiconductor layer.

Next, operations of the level shifter shown in FIG. 21 are describedwith reference to timing charts of FIGS. 22A to 22D. However, timing ofpotential change in the timing charts shown in FIGS. 22A to 22D can bedetermined as appropriate, and is not limited to that in the timingcharts of FIGS. 22A to 22D.

FIG. 23 shows an operation of the level shifter shown in FIG. 21 when anL signal is supplied to the wiring 105 and an H signal is supplied tothe wiring 106. FIG. 24 shows an operation of the level shifter shown inFIG. 21 when an H signal is supplied to the wiring 105 and an L signalis supplied to the wiring 106. Note here that the operation shown inFIG. 23 is denoted by a first operation and the operation shown in FIG.24 is denoted by a second operation.

Since operations of the offset circuit 1503 are similar to those of theoffset circuit shown in FIG. 7A, specific description thereof isomitted.

Further, since operations of a circuit including the transistors 2101and 2102 and a circuit including the transistors 2103 and 2104 aresimilar to those of the logic circuit 1500 shown in FIG. 15, specificdescription thereof is omitted.

First, the first operation of the level shifter shown in FIG. 21 isdescribed with reference to the timing charts shown in FIGS. 22A to 22Dand FIG. 23.

In the first operation, as shown in FIG. 23, an H signal is outputtedfrom the wiring 109-1 and an L signal is outputted from the wiring109-2. Note that the potential of the wiring 109-1 is, similarly to thelogic circuit 1500 shown in FIG. 15, a potential (VDD−Vth2103) obtainedby subtracting a threshold voltage Vth2103 of the transistor 2103 fromthe power source potential VDD. Further, the potential of the wiring109-2 is, similarly to the logic circuit 1500 shown in FIG. 15,determined by operating points of the transistors 2101 and 2102, thus itis a little higher than the power source potential VSS.

Next, the second operation of the level shifter shown in FIG. 21 isdescribed with reference to the timing charts shown in FIGS. 22A to 22Dand FIG. 24.

In the second operation, as shown in FIG. 24, an L signal is outputtedfrom the wiring 109-1 and an H signal is outputted from the wiring109-2. Note that the potential of the wiring 109-1 is, similarly to thelogic circuit 1500 shown in FIG. 15, determined by operating points ofthe transistors 2103 and 2104, thus it is a little higher than the powersource potential VSS. Further, the potential of the wiring 109-2 is,similarly to the logic circuit 1500 shown in FIG. 15, a potential(VDD−Vth2101) obtained by subtracting a threshold voltage Vth2101 of thetransistor 2101 from the power source potential VDD.

Here, functions of the logic circuit 2100 are described.

The logic circuit 2100 includes two logic circuits 1500 shown in FIG.15, and outputs two reversed signals through the wirings 109-1 and 109-2respectively.

Here, functions of the transistors 2101 to 2104 are described.

First, the transistor 2101 has a function as a diode. Input terminalsthereof are the gate and the first terminal, and an output terminalthereof is the second terminal. The transistor 2101 is any elementhaving a resistance component. By using a resistor instead of thetransistor 2101, the potential of the wiring 109-2 can be made equal tothe power source potential VDD in the second operation in the logiccircuit 2100.

The transistor 2102 has a function as a switch of selecting whether thewiring 104 and the wiring 109-2 are connected to each other or notdepending on the potential of the node N72. The transistor 2102 isturned on and supplies the power source potential VSS to the wiring109-2 in the first operation.

The transistor 2103 has a function as a diode. Input terminals thereofare the gate and the first terminal, and an output terminal thereof isthe second terminal. The transistor 2103 is any element having aresistance component. By using a resistor instead of the transistor2103, the potential of the wiring 109-1 can be made equal to the powersource potential VDD in the first operation in the logic circuit 2100.

The transistor 2104 has a function as a switch of selecting whether thewiring 104 and the wiring 109-1 are connected to each other or notdepending on the potential of the node N71. The transistor 2104 isturned on and supplies the power source potential VSS to the wiring109-1 in the first operation.

By the above-described first and second operations, the level shiftershown in FIG. 21 can shift each control signal supplied to the wirings105 and 106 such that the H-signal potential is shifted from VH to VDDand the L-signal potential is shifted from VL to VSS, and then outputthrough the wirings 109-1 and 109-2.

The level shifter shown in FIG. 21 can output two signals each of whichH and L levels are revered through the wirings 109-1 and 109-2respectively.

Since the gate voltage of the transistor 2102 is (VSS+(VH−VL)) in thefirst operation and VSS in the second operation, through current of thelogic circuit 2100 is reduced. This is because the amplitude voltage ofthe gate of the transistor 2102 is small (VH−VL). Therefore, powerconsumption of a semiconductor device including the level shifter shownin FIG. 21 is reduced because through current of the logic circuit 2100is small.

Similarly to the transistor 2102, since the gate voltage of thetransistor 2104 is VSS in the first operation and is (VSS+(VH−VL)) inthe second operation, through current of the logic circuit 2100 isreduced. This is because the amplitude voltage of the gate of thetransistor 2104 is small (VH−VL). Therefore, power consumption of asemiconductor device including the level shifter shown in FIG. 21 isreduced because through current of the logic circuit 2100 is small.

Further, since the amplitude voltage of the gate of the transistor 2102is small, noise generated in the logic circuit 2100 is reduced. This isbecause noise generated through parasitic capacitance between the gateand the second terminal (the wiring 109-2) of the transistor 2102becomes small.

Similarly to the transistor 2102, since the amplitude voltage of thegate of the transistor 2104 is small, noise generated in the logiccircuit 2100 is reduced. This is because noise generated throughparasitic capacitance between the gate and the second terminal (thewiring 109-1) of the transistor 2104 becomes small.

Further, it is preferable that capacitance of the capacitors 701 and 702be almost equal to each other. This is because if capacitance of thecapacitors 701 and 702 are equal to each other, timing deviations suchas output-signal delays of the wirings 109-1 and 109-2 can be equal toeach other.

Note also that as described above, a MOS capacitor may be employed aseach of the capacitors 701 and 702. Further, in the case of the levelshifter shown in FIG. 21, it is preferable to use an N-channeltransistor for each capacitor similarly to the offset circuit shown inFIG. 8A. This is because by forming each capacitor using an N-channeltransistor, amorphous silicon can be used for a semiconductor layer inthe level shifter shown in FIG. 21 so that a manufacturing process canbe simplified. Consequently, manufacturing cost can be reduced and ayield can be improved. Further, a large semiconductor device can also bemanufactured.

Further, in the level shifter shown in FIG. 21, the manufacturingprocess can be simplified even if polysilicon or single crystallinesilicon is used for the semiconductor layer.

Here, although the level shifter shown in FIG. 21 includes the N-channeltransistors and the capacitors, it may include P-channel transistors andcapacitors. FIG. 35 shows a level shifter including P-channeltransistors and capacitors.

A level shifter shown in FIG. 35 includes the capacitor 701, thecapacitor 702, the transistor 501, the transistor 502, a transistor3501, a transistor 3502, a transistor 3503, and a transistor 3504.

Note that the capacitor 301, the capacitor 302, the transistor 501, thetransistor 502, the transistor 3501, the transistor 3502, the transistor3503, and the transistor 3504 correspond to and have similar functionsto the capacitor 701, the capacitor 702, the transistor 703, thetransistor 704, the transistor 2101, the transistor 2102, the transistor2103, and the transistor 2104 in FIG. 21 respectively. The offsetcircuit 2903 and a logic circuit 3500 correspond to and have similarfunctions to the offset circuit 1503 and the logic circuit 2100 in FIG.21 respectively. The nodes N51 and N52 correspond to the nodes N71 andN72 in FIG. 21.

As shown in the level shifter in FIG. 35, a gate of the transistor 3501is connected to the wiring 104, a first terminal of the transistor 3501is connected to the wiring 104, and a second terminal of the transistor3501 is connected to the wiring 109-2. A gate of the transistor 3502 isconnected to the node N52, a first terminal of the transistor 3502 isconnected to the wiring 103, and a second terminal of the transistor3502 is connected to the wiring 109-2. A gate of the transistor 3503 isconnected to the wiring 104, a first terminal of the transistor 3503 isconnected to the wiring 104, and a second terminal of the transistor3503 is connected to the wiring 109-1. A gate of the transistor 3504 isconnected to the node N51, a first terminal of the transistor 3504 isconnected to the wiring 103, and a second terminal of the transistor3504 is connected to the wiring 109-1.

Next, operations of the level shifter shown in FIG. 35 are describedwith reference to timing charts of FIGS. 36A to 36D. However, timing ofpotential change in the timing charts shown in FIGS. 36A to 36D can bedetermined as appropriate, and is not limited to that in the timingcharts of FIGS. 36A to 36D.

FIG. 37 shows an operation of the level shifter shown in FIG. 35 when anL signal is supplied to the wiring 105 and an H signal is supplied tothe wiring 106. FIG. 38 shows an operation of the level shifter shown inFIG. 35 when an H signal is supplied to the wiring 105 and an L signalis supplied to the wiring 106. Note here that the operation shown inFIG. 37 is denoted by a first operation and the operation shown in FIG.38 is denoted by a second operation.

Since operations of the offset circuit 2903 are similar to those of theoffset circuit shown in FIG. 5A, specific description thereof isomitted.

Further, since operations of a circuit including the transistors 3501and 3502 and a circuit including the transistors 3503 and 3504 aresimilar to those of the logic circuit 2900 shown in FIG. 29, specificdescription thereof is omitted.

First, the first operation of the level shifter shown in FIG. 35 isdescribed with reference to the timing charts shown in FIGS. 36A to 36Dand FIG. 37.

In the first operation, as shown in FIG. 37, an H signal is outputtedfrom the wiring 109-1 and an L signal is outputted from the wiring109-2. Note that the potential of the wiring 109-1 is, similarly to thelogic circuit 2900 shown in FIG. 29, determined by operating points ofthe transistors 3503 and 3504, thus it is a little lower than the powersource potential VDD. Further, the potential of the wiring 109-2 is,similarly to the logic circuit 2900 shown in FIG. 29, a potential(VSS+|Vth3501|) obtained by adding an absolute value of a thresholdvoltage Vth3501 of the transistor 3501 to the power source potentialVSS.

Next, the second operation of the level shifter shown in FIG. 35 isdescribed with reference to the timing charts shown in FIGS. 36A to 36Dand FIG. 38.

In the second operation, as shown in FIG. 38, an L signal is outputtedfrom the wiring 109-1 and an H signal is outputted from the wiring109-2. Note that the potential of the wiring 109-1 is, similarly to thelogic circuit 2900 shown in FIG. 29, a potential (VSS+|Vth3503|)obtained by adding an absolute value of a threshold voltage Vth3503 ofthe transistor 3503 to the power source potential VSS. Further, thepotential of the wiring 109-2 is, similarly to the logic circuit 2900shown in FIG. 29, determined by operating points of the transistors 3501and 3502, thus it is a little lower than the power source potential VDD.

By the above-described first and second operations, the level shiftershown in FIG. 35 can shift each control signal supplied to the wirings105 and 106 such that the H-signal potential is shifted from VH to VDDand the L-signal potential is shifted from VL to VSS, and then outputthrough the wirings 109-1 and 109-2.

The level shifter shown in FIG. 35 can output two signals each of whichH and L levels are revered through the wirings 109-1 and 109-2.

Since the gate voltage of the transistor 3502 is VDD in the firstoperation and is (VDD−(VH−VL)) in the second operation, through currentof the logic circuit 3500 is reduced. This is because the amplitudevoltage of the gate of the transistor 3502 is small (VH−VL). Therefore,power consumption of a semiconductor device including the level shiftershown in FIG. 35 is reduced because through current of the logic circuit3500 is small.

Similarly to the transistor 3502, since the gate voltage of thetransistor 3504 is (VDD−(VH−VL)) in the first operation and is VDD inthe second operation, through current of the logic circuit 3500 isreduced. This is because the amplitude voltage of the gate of thetransistor 3504 is small (VH−VL). Therefore, power consumption of asemiconductor device including the level shifter shown in FIG. 35 isreduced because through current of the logic circuit 3500 is small.

Further, since the amplitude voltage of the gate of the transistor 3502is small, noise generated in the logic circuit 3500 is reduced. This isbecause noise generated through parasitic capacitance between the gateand the second terminal (the wiring 109-2) of the transistor 3502becomes small.

Similarly to the transistor 3502, since the amplitude voltage of thegate of the transistor 3504 is small, noise generated in the logiccircuit 3500 is reduced. This is because noise generated throughparasitic capacitance between the gate and the second terminal (thewiring 109-1) of the transistor 3504 becomes small.

Further, it is preferable that Capacitance of the capacitors 301 and 302be almost equal to each other. This is because if capacitance of thecapacitors 301 and 302 are equal to each other, timing deviations suchas output-signal delays of the wirings 109-1 and 109-2 can be equal toeach other.

Note also that as described above, a MOS capacitor may be employed aseach of the capacitors 301 and 302. Further, in the case of the levelshifter shown in FIG. 35, it is preferable to use a P-channel transistorfor each capacitor similarly to the offset circuit shown in FIG. 6A.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 7

This embodiment mode describes specific structures of the level shifterdescribed in Embodiment Mode 1, which are different from EmbodimentModes 5 and 6.

Note that in this embodiment mode and Embodiment Modes 1 to 6, referencenumerals are used in common, and detailed description of the sameportions and portions having similar functions is not repeated.

First, a specific structure example of a level shifter of the presentinvention is described with reference to FIG. 25.

A level shifter shown in FIG. 35 includes the capacitor 701, thecapacitor 702, the transistor 703, the transistor 704, a transistor2501, a transistor 2502, a transistor 2503, and a transistor 2504.

As shown in the level shifter in FIG. 35, a first terminal of thetransistor 2501 is connected to the wiring 103, and a second terminal ofthe transistor 2501 is connected to the wiring 109. A gate of thetransistor 2502 is connected to the node N72, a first terminal of thetransistor 2502 is connected to the wiring 104, and a second terminal ofthe transistor 2502 is connected to the wiring 109. A gate of thetransistor 2503 is connected to the wiring 103, a first terminal of thetransistor 2503 is connected to the wiring 103, and a second terminal ofthe transistor 2503 is connected to a gate of the transistor 2501. Agate of the transistor 2504 is connected to the node N72, a firstterminal of the transistor 2504 is connected to the wiring 104, and asecond terminal of the transistor 2504 is connected to the gate of thetransistor 2501. Note that a connection point of the gate of thetransistor 2501, the second terminal of the transistor 2503, and thesecond terminal of the transistor 2504 is denoted by a node N251.

A logic circuit 2500 includes the transistors 2501 to 2504. The logiccircuit 2500 corresponds to the circuit 102 shown in FIGS. 1A to 1C.

Note that the transistors 2501 to 2504 are N-channel transistors.Therefore, since the level shifter shown in FIG. 25 can be formed byusing the transistors of only the N-channel type, amorphous silicon canbe used for a semiconductor layer in the level shifter shown in FIG. 25so that a manufacturing process can be simplified. Consequently,manufacturing cost can be reduced and a yield can be improved. Further,a large semiconductor device can also be manufactured.

Further, in the level shifter shown in FIG. 25, the manufacturingprocess can be simplified even if polysilicon or single crystallinesilicon is used for the semiconductor layer.

Next, operations of the level shifter shown in FIG. 25 are, similarly tothe level shifter shown in FIG. 15, described with reference to thetiming charts of FIGS. 16A to 16C. However, timing of potential changein the timing charts shown in FIGS. 16A to 16C can be determined asappropriate, and is not limited to that in the timing charts of FIGS.16A to 16C.

FIG. 26 shows an operation of the level shifter shown in FIG. 25 when anL signal is supplied to the wiring 105 and an H signal is supplied tothe wiring 106. FIG. 27 shows an operation of the level shifter shown inFIG. 25 when an H signal is supplied to the wiring 105 and an L signalis supplied to the wiring 106. Note here that the operation shown inFIG. 26 is denoted by a first operation and the operation shown in FIG.27 is denoted by a second operation.

Since operations of the offset circuit 1503 are similar to those of theoffset circuit shown in FIG. 7A, specific description thereof isomitted.

First, the first operation of the level shifter shown in FIG. 25 isdescribed with reference to the timing charts shown in FIGS. 16A to 16Cand FIG. 26.

When the wiring 105 becomes an L level, the potential of the node N71becomes VSS. On the other hand, when the wiring 106 becomes an H level,the potential of the node N72 becomes (VSS+(VH−VL)). Consequently, thetransistors 2502 and 2504 are turned on. Since the transistor 2504 isturned on, the power source potential VSS is supplied to the node N251,so that the potential of the node N251 is decreased. Note that thepotential of the node N251 is determined by operating points of thetransistors 2503 and 2504, thus it is a little higher than the powersource potential VSS. Since the node N251 becomes an L level, thetransistor 2501 is turned off. In addition, since the transistor 2502 isturned on, the power source potential VSS is supplied to the wiring 109,so that the potential of the wiring 109 is decreased. The potential ofthe wiring 109 is decreased to the power source potential VSS, and an Lsignal is outputted from the wiring 109.

Next, the second operation of the level shifter shown in FIG. 25 isdescribed with reference to the timing charts shown in FIGS. 16A to 16Cand FIG. 27.

When the wiring 105 becomes an H level, the potential of the node N71becomes (VSS+(VH−VL)). On the other hand, when the wiring 106 becomes anL level, the potential of the node N72 becomes VSS. Consequently, thetransistors 2502 and 2504 are turned off. Since the transistor 2504 isturned off, the power source potential VDD is supplied to the node N251,so that the potential of the node N251 is increased. At the same time asthe potential increase of the node N251, the transistor 2501 is turnedon, and the power source potential VDD is supplied to the wiring 109, sothat the potential of the wiring 109 is also increased. When thepotential of the node N251 becomes a value (VDD−Vth2503) which isobtained by subtracting a threshold voltage Vth2503 of the transistor2503 from the power source potential VDD, the transistor 2503 is turnedoff and the node N251 becomes the floating state. However, the potentialincrease of the wiring 109 continues even after the potential of thenode N251 becomes (VDD−Vth2503). Therefore, the potential of the nodeN251 continues to increase by capacitive coupling of parasiticcapacitance between the gate (the node N251) and the second terminal(the wiring 109) of the transistor 2501. The potential increase of thenode N251 continues until the potential increase of the wiring 109 isstopped, so that the potential of the node N251 becomes a value equal toor higher than a value (VDD+Vth2501) which is obtained by adding athreshold voltage Vth2501 of the transistor 2501 to the power sourcepotential VDD. Note that the potential increase of the wiring 109 isstopped when the potential of the wiring 109 becomes equal to the powersource potential VDD. This is so-called bootstrap. Accordingly, thepotential of the wiring 109 becomes equal to the power source potentialVDD, and an H signal is outputted from the wiring 109.

Here, a function of the logic circuit 2500 is described.

The logic circuit 2500 has a function of selecting which one of thepower source potential VDD and the power source potential VSS issupplied to the wiring 109. In the case of supplying the power sourcepotential VDD to the wiring 109, the gate potential of the transistor2501 is made to equal to or higher than (VDD+Vth2501), thereby thepotential of the wiring 109 becomes equal to the power source potentialVDD by bootstrap.

Here, functions of the transistors 2501 to 2504 are described.

First, the transistor 2501 has a function as a switch of selectingwhether the wiring 103 and the wiring 109 are connected to each other ornot depending on the potential of the node N251. The transistor 2501 isturned on and supplies the power source potential VDD to the wiring 109in the second operation.

The transistor 2502 has a function as a switch of selecting whether thewiring 104 and the wiring 109 are connected to each other or notdepending on the potential of the node N72. The transistor 2502 suppliesthe power source potential VSS to the wiring 109 in the first operation.

The transistor 2503 has a function as a diode. Input terminals thereofare the gate and the first terminal, and an output terminal thereof isthe second terminal.

The transistor 2504 has a function as a switch of selecting whether thewiring 104 and the node N251 are connected to each other or notdepending on the potential of the node N72. The transistor 2504 suppliesthe power source potential VSS to the node N251 in the first operation.

By the above-described first and second operations, the level shiftershown in FIG. 25 can make the potential of the wiring 109 equal to thepower source potential VSS in the first operation and can make thepotential of the wiring 109 equal to the power source potential VDD inthe second operation.

Further, similarly to the logic circuit 1500 shown in FIG. 15, sinceeach amplitude voltage of the gates of the transistors 2502 and 2504 issmall, through current of the logic circuit 2500 can be reduced.

Further, similarly to the logic circuit 1500 shown in FIG. 15, sinceeach amplitude voltage of the gates of the transistors 2502 and 2504 issmall, noise generated in the logic circuit 2500 is reduced.

Note also that as described above, a MOS capacitor can be employed aseach of the capacitors 701 and 702. Further, in the case of the levelshifter shown in FIG. 25, it is preferable to use an N-channeltransistor for each capacitor similarly to the offset circuit shown inFIG. 8A. This is because by forming each capacitor using an N-channeltransistor, amorphous silicon can be used for a semiconductor layer inthe level shifter shown in FIG. 25 so that a manufacturing process canbe simplified. Consequently, manufacturing cost can be reduced and ayield can be improved. Further, a large semiconductor device can also bemanufactured.

Further, in the level shifter shown in FIG. 25, the manufacturingprocess can be simplified even if polysilicon or single crystallinesilicon is used for the semiconductor layer.

Note that as shown in FIG. 28, the gates of the transistors 2502 and2504 may also be connected to the node N71 in the level shifter shown inFIG. 25. In the case where the gates of the transistors 2502 and 2504are connected to the node N71 (FIG. 28), H and L levels of the signal(the potential) of the wiring 109 are reverse to those in the case wherethe gates of the transistors 2502 and 2504 are connected to the node N72(FIG. 25) as shown in the timing charts in FIGS. 20A to 20C. Operationsof a level shifter shown in FIG. 28 are similar to those of the levelshifter shown in FIG. 25. Therefore, whether the gates of thetransistors 2502 and 2504 are connected to the node N71 or the node N72can be determined as appropriate.

Further, although not shown, two logic circuits may also be connected toeach of the nodes N71 and N72. By using two logic circuits 2500, twosignals each of which H and L levels are revered can be outputted. Inthe case where two logic circuits are connected to each of the nodes N71and N72, similarly to the level shifter shown in FIG. 21, it ispreferable that capacitance of the capacitors 701 and 702 be almostequal to each other.

Here, although the level shifter shown in FIG. 25 includes the N-channeltransistors and the capacitors, it may include P-channel transistors andcapacitors. FIG. 39 shows a level shifter including P-channeltransistors and capacitors.

A level shifter shown in FIG. 39 includes the capacitor 301, thecapacitor 302, the transistor 501, the transistor 502, a transistor3901, a transistor 3902, a transistor 3903, and a transistor 3904.

Note that the capacitor 301, the capacitor 302, the transistor 501, thetransistor 502, the transistor 3901, the transistor 3902, the transistor3903, and the transistor 3904 correspond to and have similar functionsto the capacitor 701, the capacitor 702, the transistor 703, thetransistor 704, the transistor 2501, the transistor 2502, the transistor2503, and the transistor 2504 in FIG. 25 respectively. A logic circuit3900 and the offset circuit 2903 correspond to and have similarfunctions to the logic circuit 2500 and the offset circuit 1503 in FIG.25 respectively. The nodes N51 and N52 correspond to the nodes N71 andN72 in FIG. 25.

A gate of the transistor 3902 is connected to the node N52, a firstterminal of the transistor 3902 is connected to the wiring 103, and asecond terminal of the transistor 3902 is connected to the wiring 109. Afirst terminal of the transistor 3901 is connected to the wiring 104,and a second terminal of the transistor 3901 is connected to the wiring109. A gate of the transistor 3903 is connected to the wiring 104, afirst terminal of the transistor 3903 is connected to the wiring 104,and a second terminal of the transistor 3903 is connected to a gate ofthe transistor 3901. A gate of the transistor 3904 is connected to thenode N52, a first terminal of the transistor 3904 is connected to thewiring 103, and a second terminal of the transistor 3904 is connected tothe gate of the transistor 3901. Note that a connection point of thegate of the transistor 3901, the second terminal of the transistor 3903,and the second terminal of the transistor 3904 is denoted by a nodeN391.

Next, operations of the level shifter shown in FIG. 39 are, similarly tothe level shifter shown in FIG. 29, described with reference to thetiming charts of FIGS. 30A to 30C. However, timing of potential changein the timing charts shown in FIGS. 30A to 30C can be determined asappropriate, and is not limited to that in the timing charts of FIGS.30A to 30C.

FIG. 40 shows an operation of the level shifter shown in FIG. 39 when anL signal is supplied to the wiring 105 and an H signal is supplied tothe wiring 106. FIG. 41 shows an operation of the level shifter shown inFIG. 39 when an H signal is supplied to the wiring 105 and an L signalis supplied to the wiring 106. Note here that the operation shown inFIG. 40 is denoted by a first operation and the operation shown in FIG.41 is denoted by a second operation.

Since operations of the offset circuit 2903 are similar to those of theoffset circuit shown in FIG. 5A, specific description thereof isomitted.

First, the first operation of the level shifter shown in FIG. 39 isdescribed with reference to the timing charts shown in FIGS. 30A to 30Cand FIG. 40.

When the wiring 105 becomes an L level, the potential of the node N51becomes (VDD−(VH−VL)). On the other hand, when the wiring 106 becomes anH level, the potential of the node N52 becomes VDD. Consequently, thetransistors 3902 and 3904 are turned off. Since the transistor 3904 isturned off, the power source potential VSS is supplied to the node N391,so that the potential of the node N391 is decreased. At the same time asthe potential decrease of the node N391, the transistor 3901 is turnedon, and the power source potential VSS is supplied to the wiring 109, sothat the potential of the wiring 109 is also decreased. When thepotential of the node N391 becomes a value (VSS+|Vth3903|) which isobtained by adding an absolute value of a threshold voltage Vth3903 ofthe transistor 3903 to the power source potential VSS, the transistor3903 is turned off and the node N391 becomes the floating state.However, the potential decrease of the wiring 109 continues even afterthe potential of the node N391 becomes (VSS+|Vth3903|). Therefore, thepotential of the node N391 continues to decrease by capacitive couplingof parasitic capacitance between the gate (the node N391) and the secondterminal (the wiring 109) of the transistor 3901. The potential decreaseof the node N391 continues until the potential decrease of the wiring109 is stopped, so that the potential of the node N391 becomes a valueequal to or lower than a value (VSS+|Vth3901|) which is obtained byadding an absolute value of a threshold voltage Vth3901 of thetransistor 3901 to the power source potential VSS. Note that thepotential decrease of the wiring 109 is stopped when the potential ofthe wiring 109 becomes equal to the power source potential VSS. This isso-called bootstrap. Accordingly, the potential of the wiring 109becomes equal to the power source potential VSS, and an L signal isoutputted from the wiring 109.

Next, the second operation of the level shifter shown in FIG. 39 isdescribed with reference to the timing charts shown in FIGS. 30A to 30Cand FIG. 41.

When the wiring 105 becomes an H level, the potential of the node N51becomes VDD. On the other hand, when the wiring 106 becomes an L level,the potential of the node N52 becomes (VDD−(VH−VL)). Consequently, thetransistors 3902 and 3904 are turned on. Since the transistor 3902 isturned on, the power source potential VDD is supplied to the node N391,so that the potential of the node N391 is increased. Note that thepotential of the node N391 is determined by operating points of thetransistors 3903 and 3904, thus it is a little lower than the powersource potential VDD. Since the node N391 becomes an L level, thetransistor 3901 is turned off. In addition, since the transistor 3902 isturned on, the power source potential VDD is supplied to the wiring 109,so that the potential of the wiring 109 is increased. The potential ofthe wiring 109 is increased to the power source potential VDD, and an Hsignal is outputted from the wiring 109.

By the above-described first and second operations, the level shiftershown in FIG. 39 can make the potential of the wiring 109 equal to thepower source potential VSS in the first operation and can make thepotential of the wiring 109 equal to the power source potential VDD inthe second operation.

Further, similarly to the logic circuit 3900 shown in FIG. 39, sinceeach amplitude voltage of the gates of the transistors 3902 and 3904 issmall, through current of the logic circuit 3900 can be reduced.

Further, similarly to the logic circuit 3900 shown in FIG. 39, sinceeach amplitude voltage of the gates of the transistors 3902 and 3904 issmall, noise generated in the logic circuit 3900 is reduced.

Note also that as described above, a MOS capacitor can be employed aseach of the capacitors 301 and 302. Further, in the case of the levelshifter shown in FIG. 39, it is preferable to use a P-channel transistorfor each capacitor similarly to the offset circuit shown in FIG. 6A.

Further, as shown in FIG. 42, the gates of the transistors 3902 and 3904may also be connected to the node N51 in the level shifter shown in FIG.39. Timing charts in the case where the gates of the transistors 3902and 3904 are connected to the node N51 are shown in FIGS. 36A to 36D.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 8

This embodiment mode describes layout diagrams of the level sifter ofthe present invention.

First, a layout diagram of the level shifter shown in FIG. 15 isdescribed with reference to FIG. 43.

The layout diagram in FIG. 43 is shown in the case where a semiconductorlayer 4301, a first conductive layer 4302, and a second conductive layer4303 are formed. Note that the first conductive layer 4302 functions asa gate electrode, and the second conductive layer 4303 functions as awiring layer.

In the layout diagram shown in FIG. 43, a polycrystalline semiconductor(polysilicon) is used for the semiconductor layer 4301 of eachtransistor.

In the layout diagram shown in FIG. 43, the capacitor 701, the capacitor702, the transistor 703, the transistor 704, the transistor 1501, andthe transistor 1502 are provided. The wirings 103, 104, 105, 106, and109 are similar to those described in FIG. 15.

The transistors 703, 704, 1501, and 1502 are N-channel transistors.

The capacitor 702 is formed of the semiconductor layer 4301 and thefirst conductive layer 4302 (the gate electrode). That is, the capacitor702 functions as a MOS capacitor. Since the potential of the firstconductive layer 4302 of the capacitor 702 is higher than that of thesemiconductor layer 4301 thereof as described above, a channel is formedin a channel region of the semiconductor layer 4301. Thus the capacitor702 can obtain large capacitance.

Similarly to the capacitor 702, the capacitor 701 is formed of thesemiconductor layer 4301 and the first conductive layer 4302 (the gateelectrode). That is, the capacitor 701 also functions as a MOScapacitor. Since the potential of the first conductive layer 4302 of thecapacitor 701 is higher than that of the semiconductor layer 4301thereof as described above, a channel is formed in a channel region ofthe semiconductor layer 4301. Thus the capacitor 701 can obtain largecapacitance.

Next, a layout diagram of the level shifter shown in FIG. 29 isdescribed with reference to FIG. 44.

The layout diagram in FIG. 44 is shown in the case where a semiconductorlayer 4401, a first conductive layer 4402, and a second conductive layer4403 are formed. Note that the first conductive layer 4402 functions asa gate electrode, and the second conductive layer 4403 functions as awiring layer.

In the layout diagram shown in FIG. 44, a polycrystalline semiconductor(polysilicon) is used for the semiconductor layer 4401 of eachtransistor.

In the layout diagram shown in FIG. 44, the capacitor 301, the capacitor302, the transistor 501, the transistor 502, the transistor 2901, andthe transistor 2902 are provided. The wirings 103, 104, 105, 106, and109 are similar to those described in FIG. 29.

The transistors 501, 502, 2901, and 2902 are P-channel transistors.

The capacitor 301 is formed of the semiconductor layer 4401 and thefirst conductive layer 4402 (the gate electrode). That is, the capacitor301 functions as a MOS capacitor. Since the potential of the firstconductive layer 4402 of the capacitor 301 is lower than that of thesemiconductor layer 4401 thereof as described above, a channel is formedin a channel region of the semiconductor layer 4401. Thus the capacitor301 can obtain large capacitance.

Similarly to the capacitor 301, the capacitor 302 is formed of thesemiconductor layer 4401 and the first conductive layer 4402 (the gateelectrode). That is, the capacitor 302 also functions as a MOScapacitor. Since the potential of the first conductive layer 4402 of thecapacitor 302 is lower than that of the semiconductor layer 4401 thereofas described above, a channel is formed in a channel region of thesemiconductor layer 4401. Thus the capacitor 302 can obtain largecapacitance.

Next, a layout diagram of the level shifter shown in FIG. 15, which is adifferent example from FIG. 43, is described with reference to FIG. 45.

A layout diagram in FIG. 45 is shown in the case where the semiconductorlayer 4301, the first conductive layer 4302, and the second conductivelayer 4303 are formed. Note that the first conductive layer 4302functions as a gate electrode, and the second conductive layer 4303functions as a wiring layer.

In the layout diagram shown in FIG. 45, a polycrystalline semiconductor(polysilicon) is used for the semiconductor layer 4301 of eachtransistor.

In the layout diagram shown in FIG. 45, the capacitor 701, the capacitor702, the transistor 703, the transistor 704, the transistor 1501, andthe transistor 1502 are provided. The wirings 103, 104, 105, 106, and109 are similar to those described in FIG. 15.

The transistors 703, 704, 1501, and 1502 are N-channel transistors.

The capacitor 702 is formed of the first conductive layer 4302 and thesecond conductive layer 4303. This is because since the first conductivelayer 4302 and the second conductive layer 4303 are formed of conductivematerials, capacitance of the capacitor 702 is not changed depending ona voltage applied. Consequently, a level shifter shown in FIG. 45 can bestably operated.

The capacitor 701 is formed of the first conductive layer 4302 and thesecond conductive layer 4303. This is because since the first conductivelayer 4302 and the second conductive layer 4303 are formed of conductivematerials, capacitance of the capacitor 701 is not changed depending ona voltage applied. Consequently, the level shifter shown in FIG. 45 canbe stably operated.

Further, a first electrode of the capacitor 701 and a second electrodeof the capacitor 702 are formed of the second conductive layer 4303, anda second electrode of the capacitor 701 and a first electrode of thecapacitor 702 are formed of the first conductive layer 4302. This isbecause a layout area of the level shifter shown in FIG. 45 can bereduced. Specifically, the layout area of the level shifter shown inFIG. 45 can be made smaller in the case where the second electrode ofthe capacitor 701 is formed of the first conductive layer 4302 than inthe case where the second electrode of the capacitor 701 is formed ofthe second conductive layer 4303 because the second electrode of thecapacitor 701 is connected to a gate of the transistor 703. Similarly,the layout area of the level shifter shown in FIG. 45 can be madesmaller in the case where the second electrode of the capacitor 702 isformed of the second conductive layer 4303 than in the case where thesecond electrode of the capacitor 701 is formed of the first conductivelayer 4302 because the second electrode of the capacitor 702 isconnected to a second terminal of the transistor 703.

Next, a layout diagram of the level shifter shown in FIG. 29, which is adifferent example from FIG. 44, is described with reference to FIG. 46.

The layout diagram in FIG. 46 is shown in the case where thesemiconductor layer 4401, the first conductive layer 4402, and thesecond conductive layer 4403 are formed. Note that the first conductivelayer 4402 functions as a gate electrode, and the second conductivelayer 4403 functions as a wiring layer.

In the layout diagram shown in FIG. 46, a polycrystalline semiconductor(polysilicon) is used for the semiconductor layer 4401 of eachtransistor.

In the layout diagram shown in FIG. 46, the capacitor 301, the capacitor302, the transistor 501, the transistor 502, the transistor 2901, andthe transistor 2902 are provided. The wirings 103, 104, 105, 106, and109 are similar to those described in FIG. 29.

The transistors 501, 502, 2901, and 2902 are P-channel transistors.

The capacitor 302 is formed of the first conductive layer 4402 and thesecond conductive layer 4403. This is because since the first conductivelayer 4402 and the second conductive layer 4403 are formed of conductivematerials, capacitance of the capacitor 302 is not changed depending ona voltage applied. Consequently, a level shifter shown in FIG. 46 can bestably operated.

The capacitor 301 is formed of the first conductive layer 4402 and thesecond conductive layer 4403. This is because since the first conductivelayer 4402 and the second conductive layer 4403 are formed of conductivematerials, capacitance of the capacitor 301 is not changed depending ona voltage applied. Consequently, the level shifter shown in FIG. 46 canbe stably operated.

Further, a first electrode of the capacitor 301 and a second electrodeof the capacitor 302 are formed of the second conductive layer 4403, anda second electrode of the capacitor 301 and a first electrode of thecapacitor 302 are formed of the first conductive layer 4402. This isbecause a layout area of the level shifter shown in FIG. 46 can bereduced. Specifically, the layout area of the level shifter shown inFIG. 46 can be made smaller in the case where the second electrode ofthe capacitor 301 is formed of the first conductive layer 4402 than inthe case where the second electrode of the capacitor 301 is formed ofthe second conductive layer 4403 because the second electrode of thecapacitor 301 is connected to a gate of the transistor 501. Similarly,the layout area of the level shifter shown in FIG. 46 can be madesmaller in the case where the second electrode of the capacitor 302 isformed of the second conductive layer 4403 than in the case where thesecond electrode of the capacitor 301 is formed of the first conductivelayer 4402 because the second electrode of the capacitor 302 isconnected to a second terminal of the transistor 501.

Next, a layout diagram of the level shifter shown in FIG. 15 isdescribed with reference to FIG. 47A.

The layout diagram in FIG. 47A is shown in the case where asemiconductor layer 4701, a first conductive layer 4702, a secondconductive layer 4703, and a third conductive layer 4704 are formed.Note that the first conductive layer 4702 functions as a gate electrode,the second conductive layer 4703 functions as a wiring layer, and thethird conductive layer 4704 functions as a high-resistance wiring layer.

In the layout diagram shown in FIG. 47A, an amorphous semiconductor(amorphous silicon) is used for the semiconductor layer 4701 of eachtransistor.

In the layout diagram shown in FIG. 47A, the capacitor 701, thecapacitor 702, the transistor 703, the transistor 704, the transistor1501, and the transistor 1502 are provided. The wirings 103, 104, 105,106, and 109 are similar to those described in FIG. 15.

The capacitor 702 is formed of the semiconductor layer 4701 and thefirst conductive layer 4702 (the gate electrode). That is, the capacitor702 functions as a MOS capacitor. Since the potential of the firstconductive layer 4702 of the capacitor 702 is higher than that of thesemiconductor layer 4701 thereof as described above, a channel is formedin a channel region of the semiconductor layer 4701. Thus the capacitor702 can obtain large capacitance.

Similarly to the capacitor 702, the capacitor 701 is formed of thesemiconductor layer 4701 and the first conductive layer 4702 (the gateelectrode). That is, the capacitor 701 also functions as a MOScapacitor. Since the potential of the first conductive layer 4702 of thecapacitor 701 is higher than that of the semiconductor layer 4701thereof as described above, a channel is formed in the channel region ofthe semiconductor layer 4701. Thus the capacitor 701 can obtain largecapacitance.

Next, a layout diagram of the level shifter shown in FIG. 15, which is adifferent example from FIG. 47A, is described with reference to FIG.47B.

The layout diagram in FIG. 47B is shown in the case where thesemiconductor layer 4701, the first conductive layer 4702, the secondconductive layer 4703, and the third conductive layer 4704 are formed.Note that the first conductive layer 4702 functions as a gate electrode,the second conductive layer 4703 functions as a wiring layer, and thethird conductive layer 4704 functions as a high-resistance wiring layer.

In the layout diagram shown in FIG. 47B, an amorphous semiconductor(amorphous silicon) is used for the semiconductor layer 4701 of eachtransistor.

In the layout diagram shown in FIG. 47B, the capacitor 701, thecapacitor 702, the transistor 703, the transistor 704, the transistor1501, and the transistor 1502 are provided. The wirings 103, 104, 105,106, and 109 are similar to those described in FIG. 15.

The capacitor 702 is formed of the first conductive layer 4702 and thesecond conductive layer 4703. This is because since the first conductivelayer 4702 and the second conductive layer 4703 are formed of conductivematerials, capacitance of the capacitor 702 is not changed depending ona voltage applied. Consequently, a level shifter shown in FIG. 47B canbe stably operated.

The capacitor 701 is formed of the first conductive layer 4702 and thesecond conductive layer 4703. This is because since the first conductivelayer 4702 and the second conductive layer 4703 are formed of conductivematerials, capacitance of the capacitor 701 is not changed depending ona voltage applied. Consequently, the level shifter shown in FIG. 47B canbe stably operated.

Further, a first electrode of the capacitor 701 and a second electrodeof the capacitor 702 are formed of the second conductive layer 4703, anda second electrode of the capacitor 701 and a first electrode of thecapacitor 702 are formed of the first conductive layer 4702. This isbecause a layout area of the level shifter shown in FIG. 47B can bereduced. Specifically, the layout area of the level shifter shown inFIG. 47B can be made smaller in the case where the second electrode ofthe capacitor 701 is formed of the first conductive layer 4702 than inthe case where the second electrode of the capacitor 701 is formed ofthe second conductive layer 4703 because the second electrode of thecapacitor 701 is connected to a gate of the transistor 703. Similarly,the layout area of the level shifter shown in FIG. 47B can be madesmaller in the case where the second electrode of the capacitor 702 isformed of the second conductive layer 4703 than in the case where thesecond electrode of the capacitor 701 is formed of the first conductivelayer 4702 because the second electrode of the capacitor 702 isconnected to a second terminal of the transistor 703.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 9

Embodiment Mode 9 describes examples of a panel in which a plurality ofpixels are formed with reference to FIGS. 62A and 62B. In FIG. 62A, apanel 191 includes a pixel portion 591 including a plurality of pixels590 arranged in matrix. The pixel portion 591 can employ an activematrix structure in which a switching element such as a thin filmtransistor is provided for each pixel 590. As a display medium of eachpixel 590, either a light-emitting element such as anelectroluminescence element or a liquid crystal element may be provided.

Further, as shown in FIG. 62B, a driver circuit for driving the pixelportion 591 may also be provided over the same substrate as the pixelportion 591. In FIG. 62B, the same portions as FIG. 62A are denoted bythe same reference numerals, and description thereof is omitted. In FIG.62B, a source driver 593 and a gate driver 594 are shown as drivercircuits. The present invention is not limited to this, and a drivercircuit may be provided in addition to the source driver 593 and thegate driver 594. Such a driver circuit may be formed over a substrateand may be mounted on another substrate over which the pixel portion 591is formed. For example, the pixel portion 591 may be formed using thinfilm transistors over a glass substrate and such a driver circuit may beformed over a single crystalline substrate, and an IC chip of the drivercircuit may be connected over the glass substrate by COG (Chip On Glass)or alternatively, the IC chip may be connected over the glass substrateby TAB (Tape Automated Bonding) or connected to the glass substrate byusing a printed circuit board.

Further, such a driver circuit may also be formed using thin filmtransistors formed over the same substrate as the pixel portion 591 andin the same process as the thin film transistor included in each pixel590. A channel formation region of each thin film transistor may beformed either of a polycrystalline semiconductor or an amorphoussemiconductor.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 10

FIG. 63A shows a structure example of the pixel portion 591 shown inFIG. 62A or 62B (hereinafter referred to as a first pixel structure).The pixel portion 591 includes a plurality of source signal lines S1 toSp (p is a natural number), a plurality of scan lines G1 to Gq (q is anatural number) provided so as to intersect the plurality of sourcesignal lines S1 to Sp, and a pixel 690 which is provided for eachintersection of the source signal lines S1 to Sp and the scan lines G1to Gq.

A configuration of the pixel 690 shown in FIG. 63A is shown in FIG. 63B.FIG. 63B shows the pixel 690 formed at an intersection of one Sx (x is anatural number equal to or smaller than p) of the plurality of sourcesignal lines S1 to Sp and one Gy (y is a natural number equal to orsmaller than q) of the plurality of scan lines G1 to Gq. The pixel 690includes a first transistor 691, a second transistor 692, a capacitor693, and a light-emitting element 694. In this embodiment mode, as thelight-emitting element 694, an element which includes a pair ofelectrodes and emits light when current flows between the pair ofelectrodes is used. Further, as the capacitor 693, parasitic capacitanceof the second transistor 692, or the like may also be actively used.Each of the first transistor 691 and the second transistor 692 may beeither an N-channel transistor or a P-channel transistor. As eachtransistor for forming the pixel 690, a thin film transistor can beused.

A gate of the transistor 691 is connected to the scan line Gy, one of asource and a drain thereof is connected to the source signal line Sx,and the other of the source and the drain thereof is connected to a gateof the second transistor 692 and one electrode of the capacitor 693. Theother electrode of the capacitor 693 is connected to a node 695 to whicha potential V3 is supplied. One of a source and a drain of the secondtransistor 692 is connected to one electrode of the light-emittingelement 694, and the other of the source and the drain thereof isconnected to a node 696 to which a potential V2 is supplied. The otherelectrode of the light-emitting element 694 is connected to a node 697to which a potential V1 is supplied.

A display method of the pixel portion 591 shown in FIGS. 63A and 63B isdescribed.

One of the plurality of scan lines G1 to Gq is selected, and during thetime in which the scan line is selected, image signals are inputted intoall of the plurality of source signal lines S1 to Sp. In this manner,image signals are inputted into pixels of one row in the pixel portion591. The plurality of scan lines G1 to Gq are selected sequentially andduring each time, the similar operation is performed, so that imagesignals are inputted into all the pixels 690 in the pixel portion 591.

Described is an operation of the pixel 690 in which one Gy of theplurality of scan lines G1 to Gq is selected and an image signal isinputted from one Sx of the plurality of source signal lines S1 to Sp.When the scan line Gy is selected, the first transistor 691 is turnedon. ON state of a transistor means that the source and the drain areelectrically connected, and OFF state of the transistor means that thesource and the drain are not electrically connected. When the firsttransistor 691 becomes ON state, the image signal inputted into thesource signal line Sx is inputted into the gate of the second transistor692 through the first transistor 691. Whether the second transistor 692becomes ON state or OFF state is selected in accordance with theinputted image signal. When ON state of the second transistor 692 isselected, drain current of the second transistor 692 flows to thelight-emitting element 694 and the light-emitting element 694 emitslight.

The potential V2 and the potential V3 are kept such that a potentialdifference between them is always constant when the second transistor692 becomes ON state. The potential V2 and the potential V3 may be thesame potential. In the case where the potential V2 and the potential V3are the same potential, the node 695 and the node 696 may be connectedto the same wiring. The potential V1 and the potential V2 are set tohave a predetermined potential difference when light emission of thelight-emitting element 694 is selected. Thus current flows to thelight-emitting element 694 and the light-emitting element 694 emitslight.

Each wiring or electrode is formed of a material of the following: oneelement or a plurality of elements selected from aluminum (Al), tantalum(Ta), titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd),chromium (Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag),copper (Cu), magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn),niobium (Nb), silicon (Si), phosphorus (P), boron (B), arsenic (As),gallium (Ga), indium (In), tin (Sn), and oxygen (O); a compound or analloy material containing one element or a plurality of elementsselected from the above elements (e.g., Indium Tin Oxide (ITO), IndiumZinc Oxide (IZO), ITO containing silicon oxide (ITSO), zinc oxide (ZnO),aluminum-neodymium (Al—Nd), or magnesium-silver (Mg—Ag)); a materialcombining any of the above-described compounds; and the like. Further, acompound of silicon and any of the above-described ones (silicide)(e.g., aluminum-silicon, molybdenum-silicon, or nickel silicide) or acompound of nitrogen and any of the above-described ones (e.g., titaniumnitride, tantalum nitride, or molybdenum nitride) can also be used. Notethat silicon (Si) may contain an N-type impurity (e.g., phosphorus) or aP-type impurity (e.g., boron) at a high concentration; by containingsuch an impurity, the conductivity is improved or the similar action toany conductor is performed, thereby utilizing the silicon as a wiring oran electrode more easily. Note that any of single crystalline silicon,polycrystalline silicon (polysilicon), and amorphous silicon can be usedas the silicon. Resistance can be reduced by using single crystallinesilicon or polycrystalline silicon, and manufacturing can be performedthrough a simple manufacturing process by using amorphous silicon. Notethat, in the case of using aluminum or silver, because of its highconductivity, signal delay can be reduced, and patterning can be easilyperformed to perform microfabrication because it is easy to be etched.Further, also in the case of using copper, signal delay can be reducedbecause of its high conductivity. In the case of using molybdenum, aproblem such as a material defect does not occur in the manufacturingprocess even if molybdenum is in contact with an oxide semiconductorsuch as ITO or IZO, or silicon, patterning or etching can be performedeasily, and the heat resistance is high. In the case of using titaniumalso, a problem such as a material defect does not occur in themanufacturing process even if titanium is in contact with an oxidesemiconductor such as ITO or IZO or silicon, and the heat resistance ishigh. Further, tungsten or neodymium is also preferable because of itshigh heat resistance. In particular, an alloy of neodymium and aluminumis preferable in that the heat resistance is improved and a hillock ofaluminum can be suppressed. Further, silicon is preferable in that itcan be formed at the same time as a semiconductor layer included in atransistor and the heat resistance is high. Further, Indium Tin Oxide(ITO), Indium Zinc Oxide (IZO), ITO containing silicon oxide (ITSO),zinc oxide (ZnO), or silicon (Si) which has a light-transmittingproperty is preferable when it is used for a portion though which lightis transmitted; for example, they can be used for a pixel electrode or acommon electrode.

Note that each wiring or electrode may employ either a single layerstructure or a multi-layer structure using any of the above-describedmaterials. By employing a single layer structure, the manufacturingprocess can be simplified, the number of process days can be reduced,and cost can be reduced. Alternatively, by employing a multi-layerstructure, advantages of the materials can be utilized whiledisadvantages thereof can be decreased, thereby high-performance wiringor electrode can be formed. For example, by containing a low-resistancematerial (e.g., aluminum) in the multi-layer structure, the resistanceof the wiring can be reduced. Further, by containing a high heatresistance material in the multi-layer structure (e.g., a stacked-layerstructure in which a low heat resistance material having an advantage issandwiched using a high heat resistance material), the heat resistancecan be improved as the whole of the wiring or the electrode. Forexample, it is preferable to employ a stacked-layer structure in which alayer containing aluminum is sandwiched by layers containing molybdenumor titanium. Note that when a wiring or an electrode has a portion whichis directly in contact with a wiring or an electrode formed of anothermaterial, they may have an adverse effect on each other. For example,one material is mixed into the other material to change properties ofboth the materials, thereby, for example, an original purpose cannot beachieved or a problem occurs at the time of manufacturing so that normalmanufacturing cannot be performed. In this case, such a problem can besolved by sandwiching or covering one layer by another layer. Forexample, when Indium Tin Oxide (ITO) and aluminum are in contact witheach other, titanium or molybdenum is preferably sandwichedtherebetween. Similarly, also when silicon and aluminum are made to bein contact with each other, titanium or molybdenum is preferablysandwiched therebetween.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 11

FIG. 64A shows a structure example of the pixel portion 591 shown inFIG. 62A or 62B (hereinafter referred to as a second pixel structure),which is different from the first pixel structure described inEmbodiment Mode 10. The pixel portion 591 includes a plurality of sourcesignal lines S1 to Sp (p is a natural number), a plurality of scan linesG1 to Gq (q is a natural number) and a plurality of scan lines R1 to Rqprovided so as to intersect the plurality of source signal lines S1 toSp, and a pixel 790 which is provided for each intersection of thesource signal lines S1 to Sp and the scan lines G1 to Gq.

A configuration of the pixel 790 shown in FIG. 64A is shown in FIG. 64B.FIG. 64B shows the pixel 790 formed at an intersection of one Sx (x is anatural number equal to or smaller than p) of the plurality of sourcesignal lines S1 to Sp, one Gy (y is a natural number equal to or smallerthan q) of the plurality of scan lines G1 to Gq, and one Ry of theplurality of scan lines R1 to Rq. Note that the same portions as FIG.63B are denoted by the same reference numerals in the pixel structureshown in FIG. 64B, and description thereof is omitted. The pixel 790 inFIG. 64B is different from the pixel 690 in FIG. 63B in that a thirdtransistor 791 is further provided. The third transistor 791 may beeither an N-channel transistor or a P-channel transistor. As eachtransistor for forming the pixel 790, a thin film transistor can beused.

A gate of the third transistor 791 is connected to the scan line Ry, oneof a source and a drain thereof is connected to a gate of the secondtransistor 692 and one electrode of the capacitor 693, and the other ofthe source and the drain thereof is connected to a node 792 to which apotential V4 is supplied.

A display method of the pixel portion 591 shown in FIGS. 64A and 64B isdescribed.

A method by which the light-emitting element 694 emits light is the sameas that described in Embodiment Mode 10. In accordance with the pixelstructure shown in FIGS. 64A and 64B, since the scan line Ry and thethird transistor 791 are provided, no-light emission state of thelight-emitting element 694 can be obtained regardless of the imagesignal imputed from the source line Sx. By a signal inputted into thescan line Ry, time for light emission of the light-emitting element 694in the pixel 790 can be set. In this manner, a light-emitting periodwhich is shorter than a period during which all the scan lines G1 to Gqare selected sequentially can be set. In this manner, a short subframeperiod can be set in the case where display is performed by atime-division gray scale method, thereby display in a high gray-scalelevel can be performed.

The potential V4 may be set such that the second transistor 692 becomesOFF state when the third transistor 791 becomes ON state. For example,the potential V4 can be set such that it becomes equal to the potentialV3 when the third transistor 791 becomes ON state. By setting thepotentials V3 and V4 to the same potential, electrical charges held inthe capacitor 693 are discharged, and a gate-source voltage of thesecond transistor 692 becomes zero, thereby turning off the secondtransistor 692. In the case where the potential V3 and the potential V4are the same potential, the node 695 and the node 792 may be connectedto the same wiring.

Note that an arrangement of the third transistor 791 is not limited tothat shown in FIG. 64B. For example, the third transistor 791 may alsobe provided in series with the second transistor 692. In such aconfiguration, current to flow into the light-emitting element 694 isshut off and no-light emission state of the light-emitting element 694can be obtained by making the third transistor 791 OFF state with asignal inputted into the scan line Ry.

A diode can also be used instead of the third transistor 791 shown inFIG. 64B. FIG. 64C shows a configuration of a pixel in which a diode isused instead of the third transistor 791. Note that the same portions asFIG. 64B are denoted by the same reference numerals in the pixelstructure shown in FIG. 64C, and description thereof is omitted. Oneelectrode of a diode 781 is connected to the scan line Ry, and the otherelectrode thereof is connected to a gate of the second transistor 692and one electrode of the capacitor 693.

In the diode 781, current flows from one electrode to the otherelectrode. The second transistor 692 is a P-channel transistor. Byincreasing the potential of one electrode of the diode 781, thepotential of the gate of the second transistor 692 is increased so thatthe second transistor 692 can become OFF state.

Although current flows from one electrode of the diode 781 connected tothe scan line Ry to the other electrode thereof connected to the gate ofthe second transistor 692 and the second transistor 692 is a P-channeltransistor, the present invention is not limited to this. Aconfiguration in which current flows from the other electrode of thediode 781 connected to the gate of the second transistor 692 to the oneelectrode thereof connected to the scan line Ry and the secondtransistor 692 is an N-channel transistor may also be used. In the casewhere the second transistor 692 is an N-channel transistor, bydecreasing the potential of the one electrode of the diode 781, thepotential of the gate of the second transistor 692 is decreased so thatthe second transistor 692 can become OFF state.

As the diode 781, a diode-connected transistor may also be used. Thediode-connected transistor means a transistor with the gate and thedrain connected to each other. As the diode-connected transistor, eithera P-channel transistor or an N-channel transistor may be used.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 12

FIG. 65A shows a structure example of the pixel portion 591 shown inFIG. 62A or 62B (hereinafter referred to as a third pixel structure).The pixel portion 591 includes a plurality of source signal lines S1 toSp (p is a natural number), a plurality of scan lines G1 to Gq (q is anatural number) provided so as to intersect the plurality of sourcesignal lines S1 to Sp, and the pixel 690 which is provided for eachintersection of the source signal lines S1 to Sp and the scan lines G1to Gq.

A configuration of the pixel 690 shown in FIG. 65A is shown in FIG. 65B.FIG. 65B shows the pixel 690 formed at an intersection of one Sx (x is anatural number equal to or smaller than p) of the plurality of sourcesignal lines S1 to Sp and one Gy (y is a natural number equal to orsmaller than q) of the plurality of scan lines G1 to Gq. Further, acapacitance line C0 is provided for each row. The pixel 690 includes atransistor 4691, a liquid crystal element 4692, and a capacitor 4693.The transistor 4691 may be either an N-channel transistor or a P-channeltransistor. As each transistor for forming the pixel 690, a thin filmtransistor can be used.

A gate of the transistor 4691 is connected to the scan line Gy, one of asource and a drain thereof is connected to the source signal line Sx,and the other of the source and the drain thereof is connected to oneelectrode of the liquid crystal element 4692 and one electrode of thecapacitor 4693. The other electrode of the liquid crystal element 4692is connected to a node 4694 to which a potential V0 is supplied. Theother electrode of the capacitor 4693 is connected to the capacitanceline C0. To the capacitance line C0, the same potential as the potentialV0 supplied to the node 4694 is supplied.

A display method of the pixel portion 591 shown in FIGS. 65A and 65B isdescribed.

One of the plurality of scan lines G1 to Gq is selected, and during thetime in which the scan line is selected, image signals are inputted intoall of the plurality of source signal lines S1 to Sp. In this manner,image signals are inputted into pixels of one row in the pixel portion591. The plurality of scan lines G1 to Gq are selected sequentially andduring each time, the similar operation is performed, so that imagesignals are inputted into all the pixels 690 in the pixel portion 591.

Described is an operation of the pixel 690 in which one Gy of theplurality of scan lines G1 to Gq is selected and an image signal isinputted from one Sx of the plurality of source signal lines S1 to Sp.When the scan line Gy is selected, the transistor 4691 becomes ON state.ON state of a transistor means that the source and the drain areelectrically connected, and OFF state of the transistor means that thesource and the drain are not electrically connected. When the transistor4691 becomes ON state, the image signal inputted into the source signalline Sx is inputted into the one electrode of the liquid crystal element4692 and the one electrode of the capacitor 4693 through the transistor4691. In this manner, a voltage (which corresponds to a potentialdifference between the potential of the inputted image signal and thepotential V0 of the node 4694) is applied between the pair of electrodesof the liquid crystal element 4692, thereby transmittance of the liquidcrystal element 4692 is changed.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 13

This embodiment mode describes an example of a pixel actuallymanufactured. Each of FIGS. 48A and 48B is a cross-sectional diagram ofa pixel of the panel described in each of Embodiment Modes 11 and 12. Anexample of a light-emitting device in which a TFT is used as a switchingelement disposed in the pixel and a light-emitting element is used as adisplay medium disposed in the pixel is shown.

In FIGS. 48A and 48B, 1000 denotes a substrate, 1001 denotes a basefilm, 1002 denotes a semiconductor layer, 1102 denotes a semiconductorlayer, 1003 denotes a first insulating film, 1004 denotes a gateelectrode, 1104 denotes an electrode, 1005 denotes a second insulatingfilm, 1006 denotes an electrode, 1007 denotes a first electrode, 1008denotes a third insulating film, 1009 denotes a light-emitting layer,and 1010 denotes a second electrode. Reference numeral 1100 denotes aTFT, 1011 denotes a light-emitting element, and 1101 denotes acapacitor. In FIGS. 48A and 48B, the TFT 1100 and the capacitor 1101 aretypically shown as elements for forming each pixel. A structure shown inFIG. 48A is described.

As the substrate 1000, a glass substrate made of barium borosilicateglass, alumino borosilicate glass, or the like, a quartz substrate, aceramic substrate, or the like can be used. Further, a metal substratecontaining stainless steel or a semiconductor substrate with aninsulating film formed thereon may also be used. A substrate formed of aflexible synthetic resin such as plastic may also be used. A surface ofthe substrate 1000 may be planarized by polishing such as CMP.

As the base film 1001, an insulating film of silicon oxide, siliconnitride, silicon nitride oxide, or the like can be used. The base film1001 can prevent diffusion of alkaline metals such as Na or alkalineearth metals contained in the substrate 1000 into the semiconductorlayer 1002, which would otherwise adversely affect the characteristicsof the TFT 1100. Although the base film 1001 employs a single layerstructure in FIGS. 48A and 48B, it may employ a multi-layer structure oftwo or more layers. Note that the base film 1001 is not necessarilyprovided in the case where diffusion of impurities does not matter, suchas in the case of using a quartz substrate.

As the semiconductor layer 1002 and the semiconductor layer 1102,patterned crystalline semiconductor film or amorphous semiconductor filmcan be used. The crystalline semiconductor film can be obtained bycrystallizing an amorphous semiconductor film. As the crystallizationmethod, laser crystallization, thermal crystallization using RTA or anannealing furnace, thermal crystallization using metal elements whichpromote crystallization, or the like can be used. The semiconductorlayer 1002 includes a channel formation region and a pair of impurityregions doped with impurity elements which impart conductivity type.Note that impurity regions (LDD regions) which are doped with theaforementioned impurity elements at a lower concentration may also beprovided between the channel formation region and the pair of impurityregions. The semiconductor layer 1102 may be entirely doped withimpurity elements which impart conductivity type.

The first insulating film 1003 can be formed of silicon oxide, siliconnitride, silicon nitride oxide, or the like, and either a single layerstructure or a stacked-layer structure can be employed.

Note that the first insulating film 1003 may be formed of a filmcontaining hydrogen so as to hydrogenate the semiconductor layer 1002.

The gate electrode 1004 and the electrode 1104 may be formed of one kindof element selected from Ta, W, Ti, Mo, Al, Cu, Cr, and Nd, or an alloyor a compound containing such elements. Further, either a single layerstructure or a stacked-layer structure thereof can be employed.

The TFT 1100 includes the semiconductor layer 1002, the gate electrode1004, and the first insulating film 1003 sandwiched between thesemiconductor layer 1002 and the gate electrode 1004. Although only theTFT 1100 connected to the first electrode 1007 of the light-emittingelement 1011 is shown as a TFT for forming each pixel in FIGS. 48A and48B, a plurality of TFTs may also be provided. Further, although the TFT1100 is described as a top-gate transistor in this embodiment mode, theTFT 1100 may also be either a bottom-gate transistor having a gateelectrode below a semiconductor layer, or a dual-gate transistor havinggate electrodes above and below a semiconductor layer.

The capacitor 1101 is formed to have the first insulating film 1003 as adielectric, and a pair of electrodes which are the semiconductor layer1102 and the electrode 1104 facing each other with the first insulatingfilm 1003 interposed therebetween. Although the semiconductor layer 1102which is formed at the same time as the semiconductor layer 1002 of theTFT 1100 is used as one of the pair of electrodes of the capacitor andthe electrode 1104 which is formed at the same time as the gateelectrode 1004 of the TFT 1100 is used as the other electrode of thecapacitor included in each pixel shown in FIGS. 48A and 48B, the presentinvention is not limited to this structure.

As the second insulating film 1005, a single layer or a stacked layerusing an inorganic insulating film or an organic insulating film can beused. As the inorganic insulating film, a silicon oxide film formed byCVD, a silicon oxide film formed by SOG (Spin On Glass), or the like canbe used. As the organic insulating film, a film of polyimide, polyamide,BCB (benzocyclobutene), acrylic, a positive photosensitive organicresin, a negative photosensitive organic resin, or the like can be used.

Further, the second insulating film 1005 may also be formed of amaterial having a skeletal structure with a bond of silicon (Si) andoxygen (O). As a substituent of such a material, an organic groupcontaining at least hydrogen (e.g., an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group may be used as thesubstituent or both the fluoro group and the organic group containing atleast hydrogen may be used as the substituent.

Note that a surface of the second insulating film 1005 may be nitridedby high-density plasma treatment. High-density plasma is generated byusing microwaves with a high frequency, e.g., 2.45 GHz. Note that as thehigh-density plasma, plasma with an electron density of 1×10¹¹ cm⁻³ ormore and an electron temperature of equal to or more than 0.2 eV andequal to or less than 2.0 eV (preferably, equal to or more than 0.5 eVand equal to or less than 1.5 eV) is used. Since the high-density plasmawhich is low in electron temperature as described above has low kineticenergy of active species, a less defective film with little plasmadamage can be formed as compared with that formed by conventional plasmatreatment. In performing high-density plasma treatment, the substrate1000 is set at a temperature of 350 to 450° C. In addition, the distancebetween an antenna for generating microwaves and the substrate 1000 inan apparatus for generating high-density plasma is set to 20 to 80 mm(preferably, 20 to 60 mm).

The surface of the second insulating film 1005 is nitrided by performingthe above-described high-density plasma treatment under an atmospherecontaining nitrogen (N) and a rare gas (containing at least one of He,Ne, Ar, Kr, and Xe), an atmosphere containing nitrogen, hydrogen (H),and a rare gas, or an atmosphere containing NH₃ and a rare gas. In thesurface of the second insulating film 1005 formed by such nitridationtreatment with high-density plasma, elements such as H, He, Ne, Ar, Kr,or Xe are mixed. For example, by using a silicon oxide film or a siliconoxynitride film as the second insulating film 1005 and treating thesurface of the film with high-density plasma, a silicon nitride film isformed. Hydrogen contained in the silicon nitride film formed in thismanner may be used for hydrogenating the semiconductor layer 1002 of theTFT 1100. Note that this hydrogenation treatment may be combined withthe above-described hydrogenation treatment using hydrogen contained inthe first insulating film 1003.

Note that an insulating film may be further formed over the nitride filmformed by the high-density plasma treatment, for forming the secondinsulating film 1005.

The electrode 1006 can be formed of one kind of elements selected fromAl, W, Mo, Ti, Pt, Cu, Ta, Au, and Mn, or an alloy containing two ormore kinds of elements selected from Al, Ni, C, W, Mo, Ti, Pt, Cu, Ta,Au, and Mn. Further, either a single layer structure or a stacked-layerstructure can be employed.

One or both of the first electrode 1007 and the second electrode 1010can be formed as a light-transmissive electrode. The light-transmissiveelectrode can be formed of indium oxide containing tungsten oxide (IWO),indium oxide containing tungsten oxide and zinc oxide (IWZO), indiumoxide containing titanium oxide (ITiO), indium tin oxide containingtitanium oxide (ITTiO), or the like. Needless to say, indium tin oxide(ITO), indium zinc oxide (IZO), indium tin oxide doped with siliconoxide (ITSO), or the like may also be used.

Light-emitting elements are classified into a light-emitting element towhich a DC voltage is applied to emit light (hereinafter referred to asa DC drive light-emitting element) and a light-emitting element to whichan AC voltage is applied to emit light (hereinafter referred to as an ACdrive light-emitting element).

In a DC drive light-emitting element, a light-emitting layer ispreferably formed of a plurality of layers having different functions,such as a hole injecting/transporting layer, a light-emitting layer, andan electron injecting/transporting layer.

The hole injecting/transporting layer is preferably formed of acomposite material of an organic compound material having a holetransporting property and an inorganic compound material which exhibitsan electron accepting property with respect to the organic compoundmaterial. By employing such a structure, a large number of hole carriersare generated in the organic compound which inherently has few carriers,thereby an extremely excellent hole injecting/transporting property canbe obtained. Due to such an effect, a driving voltage can be reducedthan in a conventional structure. Further, since the holeinjecting/transporting layer can be formed thick without increasing thedriving voltage, short circuits of the light-emitting element due todust or the like can be also suppressed.

As an organic compound material having a hole transporting property,there are the following, and the present invention is not limited tothis: 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA); 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviation: m-MTDAB);N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(abbreviation: TPD); 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB); or the like.

As an inorganic compound material which exhibits an electron acceptingproperty, there are the following: titanium oxide, zirconium oxide,vanadium oxide, molybdenum oxide, tungsten oxide, rhenium oxide,ruthenium oxide, zinc oxide, or the like. In particular, vanadium oxide,molybdenum oxide, tungsten oxide, and rhenium oxide are preferable sincethey can be deposited in vacuum and are easy to be handled.

The electron injecting/transporting layer is formed of an organiccompound material having an electron transporting property.Specifically, there are the following, and the present invention is notlimited to this: tris(8-quinolinolato)aluminum (abbreviation: Alq₃);tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃); or thelike.

In the DC drive light-emitting element, the light-emitting layer can beformed of the following: 9,10-di(2-naphthyl)anthracene (abbreviation:DNA); 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation:t-BuDNA); 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi);coumarin 30; coumarin 6; coumarin 545; coumarin 545T; perylene; rubrene;periflanthene; 2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP);9,10-diphenylanthracene (abbreviation: DPA); 5,12-diphenyltetracene;4-(dicyanomethylene)-2-methyl-[p-(dimethylamino)styryl]-4H-pyran(abbreviation: DCM1);4-(dicyanomethylene)-2-methyl-6-[2-(julolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCM2);4-(dicyanomethylene)-2,6-bis[p-(dimethylamino)styryl]-4H-pyran(abbreviation: BisDCM); or the like. Alternatively, the followingcompounds capable of generating phosphorescence can be used:bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(picolinate)(abbreviation: FIrpic);bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(picolinate)(abbreviation: Ir(CF₃ ppy)₂(pic));tris(2-phenylpyridinato-N,C^(2′))iridium (abbreviation: Ir(ppy)₃);bis(2-phenylpyridinato-N,C^(2′))iridium(acetylacetonate) (abbreviation:Ir(ppy)₂(acac));bis[2-(2′-thienyl)pyridinato-N,C^(3′)]iridium(acetylacetonate)(abbreviation: Ir(thp)₂(acac));bis(2-phenylquinolinato-N,C^(2′))iridium(acetylacetonate) (abbreviation:Ir(pq)₂(acac));bis[2-(2′-benzothienyl)pyridinato-N,C^(3′)]iridium(acetylacetonate)(abbreviation: Ir(btp)₂(acac)); or the like.

Further alternatively, the light-emitting layer can be formed of anelectroluminescent polymeric material such as apolyparaphenylene-vinylene-based material, a polyparaphenylene-basedmaterial, a polythiophene-based material, or a polyfluorene-basedmaterial.

The other of the first electrode 1007 and the second electrode 1010 maybe formed of a material which does not transmit light. For example, itmay be formed of an alkaline metal such as Li or Cs, an alkaline earthmetal such as Mg, Ca, or Sr, an alloy containing any of them (e.g.,MgAg, AlLi, or MgIn), a compound containing any of them (e.g., CaF₂ orcalcium nitride), or a rare earth metal such as Yb or Er.

The third insulating film 1008 can be formed of a similar material tothe second insulating film 1005. The third insulating film 1008 isformed on the periphery of the first electrode 1007 so as to cover endportions of the first electrode 1007, and has a function of separatingthe light-emitting layer 1009 per pixel.

The light-emitting layer 1009 is formed of a single layer or a pluralityof layers. In the case where the light-emitting layer 1009 is formed ofa plurality of layers, the layers can be classified into a holeinjecting layer, a hole transporting layer, a light-emitting layer, anelectron transporting layer, an electron injecting layer, or the like,in terms of the carrier transporting properties. Note that each boundarybetween adjacent layers is not necessarily clear, and there may be thecase where materials forming the adjacent layers are partially mixedwith each other, which makes the interface between the adjacent layersunclear. Each layer can be formed of an organic material or an inorganicmaterial. As the organic material, either a high molecular material or alow molecular material can be used.

The light-emitting element 1011 is formed of the light-emitting layer1009, the first electrode 1007 and the second electrode 1010 whichoverlap each other with the light-emitting element 1009 interposedtherebetween. One of the first electrode 1007 and the second electrode1010 corresponds to an anode, and the other thereof corresponds to acathode. The light-emitting element 1011 emits light when a forward-biasvoltage which is higher than the threshold voltage is applied betweenthe anode and the cathode and current flows from the anode to thecathode.

On the other hand, in the case of an AC drive light-emitting element, adouble-insulating structure in which a light-emitting layer sandwichedbetween two insulating films is provided between a pair of electrodes isemployed, and light emission can be obtained by applying AC voltagebetween the pair of electrodes. In the AC drive light-emitting element,the light-emitting layer can be formed of ZnS, SrS, BaAl₂S₄, or thelike. Each insulating film by which the light-emitting layer issandwiched can be formed of Ta₂O₅, SiO₂, Y₂O₃, BaTiO₃, SrTiO₃, siliconnitride, or the like.

A structure of FIG. 48B is described next. Note that the same portionsas FIG. 48A are denoted by the same reference numerals, and descriptionthereof is omitted.

FIG. 48B shows a structure in which an insulating film 1108 is providedbetween the second insulating film 1005 and the third insulating film1008 in FIG. 48A. The electrode 1006 and the first electrode 1007 areconnected by an electrode 1106 in a contact hole formed in theinsulating film 1108.

Note that the electrode 1106 is not necessarily provided. That is, thefirst electrode 1007 may also be connected directly to the electrode1006 without providing the electrode 1106. Thus a process for formingthe electrode 1106 can be omitted, thereby cost can be reduced.

In the case where the first electrode 1007 is connected directly to theelectrode 1006 without providing the electrode 1106, however, coverageof the first electrode 1007 may be deteriorated to be broken dependingon a material or a manufacturing method of the first electrode 1007. Insuch a case, it is advantageous that the electrode 1006 and the firstelectrode 1007 are connected by the electrode 1106 in the contact holeformed in the insulating film 1108 as is in FIG. 48B.

The insulating film 1108 can employ a similar structure to the secondinsulating film 1005. The electrode 1106 can employ a similar structureto the electrode 1006.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 14

This embodiment mode describes an example of a pixel actuallymanufactured. FIG. 49 is a cross-sectional diagram of a pixel of thepanel described in each of Embodiment Modes 9 to 11. An example of alight-emitting device in which a TFT is used as a switching elementdisposed in the pixel and a light-emitting element is used as a displaymedium disposed in the pixel is shown. Note that the same portions asFIGS. 48A and 48B described in Embodiment Mode 13 are denoted by thesame reference numerals, and description thereof is omitted.

A pixel shown in FIG. 49 is different from the structure shown in FIG.48A in Embodiment Mode 13, in each structure of the TFT 1100 and thecapacitor 1101. In the example, a bottom-gate TFT is used as the TFT1100. The TFT 1100 includes a gate electrode 2703, a semiconductor layerincluding a channel formation region 2706, LDD regions 2707, andimpurity regions 2708, and a first insulating film 2705 between the gateelectrode 2703 and the semiconductor layer. The first insulating film2705 functions as a gate insulating film of the TFT 1100. The impurityregions 2708 become a source region and a drain region of the TFT 1100.

The capacitor 1101 is formed to have the first insulating film 2705 as adielectric, and a pair of electrodes which are the semiconductor layer1102 and the electrode 2704 facing each other with the first insulatingfilm 2705 interposed therebetween. The semiconductor layer includes achannel formation region 2709, LDD regions 2710, and impurity regions2711. Although the semiconductor layer which is formed at the same timeas the semiconductor layer which becomes an active layer of the TFT 1100is used as one of the pair of electrodes of the capacitor and theelectrode 2704 which is formed at the same time as the gate electrode2703 of the TFT 1100 is used as the other electrode of the capacitorincluded in each pixel shown in FIG. 49, the present invention is notlimited to this structure.

Each of the semiconductor layer including the channel formation region2706, the LDD regions 2707, and the impurity regions 2708, and thesemiconductor layer including the channel formation region 2709, the LDDregions 2710, and the impurity regions 2711 can be formed of the samematerial as the semiconductor layer 1002 or the semiconductor layer 1102in FIGS. 48A and 48B. The first insulating film 2705 can be formed ofthe same material as the first insulating film 1003 in FIGS. 48A and48B. Each of the gate electrode 2703 and the electrode 2704 can beformed of the same material as the gate electrode 1004 in FIGS. 48A and48B.

Each of the channel formation regions 2706 and 2709 may also be dopedwith impurity elements which impart conductivity type.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 15

This embodiment mode describes an example of a pixel actuallymanufactured. FIG. 50 is a cross-sectional diagram of a pixel of thepanel described in Embodiment Mode 11. An example of a light-emittingdevice in which a TFT is used as a switching element disposed in thepixel and a light-emitting element is used as a display medium disposedin the pixel is shown. Note that the same portions as FIGS. 48A and 48Bdescribed in Embodiment Mode 13 are denoted by the same referencenumerals, and description thereof is omitted.

Each pixel shown in FIGS. 50A and 50B is different from the structureshown in FIG. 48A in Embodiment Mode 13, in each structure of the TFT1100 and the capacitor 1101. In an example shown in FIG. 50A, abottom-gate TFT of a channel etch type is used as the TFT 1100. In anexample shown in FIG. 50B, a bottom-gate TFT of a channel protectiontype is used as the TFT 1100. The TFT of a channel protection type 1100shown in FIG. 50B is different from the TFT of a channel etch type 1100shown in FIG. 50A, in that an insulator 3001 to be an etching mask isprovided over a region for forming a channel in a semiconductor film2906.

In FIGS. 50A and 50B, the TFT 1100 includes a gate electrode 2993, afirst insulating film 2905 over the gate electrode 2993, thesemiconductor layer 2906 over the first insulating film 2905, and N-typesemiconductor layers 2908 and 2909 over the semiconductor layer 2906.The first insulating film 2905 functions as a gate insulating film ofthe TFT 1100. The N-type semiconductor layers 2908 and 2909 become asource and a drain of the TFT 1100. Over the N-type semiconductor layers2908 and 2909, electrodes 2911 and 2912 are formed. One end portion ofthe electrode 2911 is extended to a region where the semiconductor layer2906 does not exist, and an electrode 1006 is formed in the region wherethe semiconductor layer 2906 does not exist, so as to be in contact witha top portion of the electrode 2911.

The capacitor 1101 is formed to have the first insulating film 2905 as adielectric, an electrode 2904 as one electrode, a semiconductor layer2907 which faces the electrode 2904 with the first insulating film 2905interposed therebetween, an N-type semiconductor layer 2910 over thesemiconductor layer 2907, and an electrode 2913 over the N-typesemiconductor layer 2910 as the other electrode. The electrode 2904 canbe formed at the same time as the gate electrode 2993. The semiconductorlayer 2907 can be formed at the same time as the semiconductor layer2906. The N-type semiconductor layer 2910 can be formed at the same timeas the N-type semiconductor layers 2908 and 2909. The electrode 2913 canbe formed at the same time as the electrodes 2911 and 2912.

Each of the gate electrode 2993 and the electrode 2904 can be formed ofthe same material as the gate electrode 1004 in FIGS. 48A and 48B. Aseach of the semiconductor layers 2906 and 2907, an amorphoussemiconductor film can be used. The first insulating film 2905 can beformed of the same material as the first insulating film 1003 in FIGS.48A and 48B. Each of the electrodes 2911, 2912, and 2913 can be formedof the same material as the electrode 1006. As each of the N-typesemiconductor layers 2910, 2908, and 2909, a semiconductor filmcontaining N-type impurities can be used.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 16

This embodiment mode describes an example of a pixel actuallymanufactured. FIGS. 51A to 51C are cross-sectional diagrams of a pixelof the panel described in Embodiment Mode 11. An example in which a TFTis used as a switching element disposed in the pixel and a liquidcrystal element is used as a display medium disposed in the pixel isshown.

In each pixel shown in FIGS. 51A to 51C, a liquid crystal element isprovided instead of the light-emitting element 1011 in each of thestructures shown in FIGS. 48A and 48B described in Embodiment Mode 13and the structure shown in FIG. 49 described in Embodiment Mode 14. Notethat the same portions as FIGS. 48A, 48B, and 49 are denoted by the samereference numerals, and description thereof is omitted.

The liquid crystal element is formed of a first electrode 4000, analignment film 4001 formed over the first electrode 4000, liquidcrystals 4002, an alignment film 4003, and a second electrode 4004. Byapplying a voltage between the first electrode 4000 and the secondelectrode 4004, alignment of the liquid crystals is changed so thattransmittance of the liquid crystal element is changed. The secondelectrode 4004 and the alignment film 4003 are provided for a countersubstrate 4005.

One or both of the first electrode 4000 and the second electrode 4004can be formed as a light-transmissive electrode. The light-transmissiveelectrode can be formed of indium oxide containing tungsten oxide (IWO),indium oxide containing tungsten oxide and zinc oxide (IWZO), indiumoxide containing titanium oxide (ITiO), indium tin oxide containingtitanium oxide (ITTiO), or the like. Needless to say, indium tin oxide(ITO), indium zinc oxide (IZO), indium tin oxide doped with siliconoxide (ITSO), or the like may also be used. The other of the firstelectrode 4000 and the second electrode 4004 may also be formed of amaterial which does not transmit light. For example, it may be formed ofan alkaline metal such as Li or Cs, an alkaline earth metal such as Mg,Ca, or Sr, an alloy containing any of them (e.g., MgAg, AlLi, or MgIn),a compound containing any of them (e.g., CaF₂ or calcium nitride), or arare earth metal such as Yb or Er.

As the liquid crystals 4002, known liquid crystals can be used asappropriate. For example, ferroelectric liquid crystals oranti-ferroelectric liquid crystals may be used as the liquid crystals4002. Further, as a driving mode of the liquid crystals, a TN (TwistedNematic) mode, a MVA (Multi-domain Vertical Alignment) mode, an ASM(Axially Symmetric aligned Micro-cell) mode, an OCB (OpticallyCompensated Bend) mode, or the like can be used as appropriate.

Although the pair of electrodes for applying voltage to the liquidcrystals 4002 (the first electrode 4000 and the second electrode 4004)are formed over different substrates, the present invention is notlimited to this. The second electrode 4004 may be formed over thesubstrate 1000, and then an IPS (In-Plane-Switching) mode may be used asthe driving mode of the liquid crystals. Further, either one or both ofthe alignment films 4001 and 4003 is not necessarily provided dependingon the liquid crystals 4002.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 17

This embodiment mode describes an example of a pixel actuallymanufactured.

FIGS. 52A and 52B are cross-sectional diagrams of a pixel of the paneldescribed in Embodiment Mode 13. An example in which a TFT is used as aswitching element disposed in the pixel and a liquid crystal element isused as a display medium disposed in the pixel is shown.

In each pixel shown in FIGS. 52A and 52B, a liquid crystal element isprovided instead of the light-emitting element 1011 in each structureshown in FIGS. 50A and 50B in Embodiment Mode 15. Note that the sameportions as FIGS. 50A and 50B are denoted by the same referencenumerals, and description thereof is omitted. Further, a structure ofthe liquid crystal element, and the like are similar to those in eachstructure shown in FIGS. 51A to 50C described in Embodiment Mode 16,thus description thereof is omitted.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 18

This embodiment mode describes a structure of sealing of a substrateincluding pixels with reference to FIGS. 53A to 53C. FIG. 53A is a topdiagram of a panel formed by sealing a substrate including pixels, andFIGS. 53B and 53C are cross-sectional diagrams taken along a line A-A′in FIG. 53A. A sealing method is different in FIGS. 53B and 53C.

In FIGS. 53A to 53C, a pixel portion 1302 having a plurality of pixelsis provided over a substrate 1301, and a sealing material 1306 isprovided to surround the pixel portion 1302, and a sealing material 1307is attached thereto. For a structure of each pixel, the structuredescribed in Embodiment Mode 14, 15, or 16 can be employed.

In a display panel in FIG. 53B, the sealing material 1307 in FIG. 53Acorresponds to a counter substrate 1321. The counter substrate 1321which is transparent is attached using the sealing material 1306 as anadhesive layer, thereby a hermetically-sealed space 1322 is formed bythe substrate 1301, the counter substrate 1321, and the sealing material1306. The counter substrate 1321 is provided with a color filter 1320and a protective film 1323 for protecting the color filter. Lightemitted from light-emitting elements which are disposed in the pixelportion 1302 is discharged to outside through the color filters 1320.The hermetically-sealed space 1322 is filled with an inert resin,liquid, or the like. Note that as the resin for filling thehermetically-sealed space 1322, a light-transmissive resin in which amoisture absorbent is dispersed may be used. Further, the same materialsmay be used for the sealing material 1306 and for filling thehermetically-sealed space 1322, so that the adhesion of the countersubstrate 1321 and the sealing of the pixel portion 1302 may beperformed at the same time.

In a display panel shown in FIG. 53C, the sealing material 1307 in FIG.53A corresponds to a sealing material 1324. The sealing material 1324 isattached using the sealing material 1306 as an adhesive layer, thereby ahermetically-sealed space 1308 is formed by the substrate 1301, thesealing material 1306, and the sealing material 1324. The sealingmaterial 1324 is provided with a moisture absorbent 1309 in advance inits depressed portion, and the moisture absorbent 1309 functions to keepa clean atmosphere in the hermetically-sealed space 1308 by adsorbingmoisture, oxygen, or the like, and to suppress degradation of thelight-emitting elements. The depressed portion is covered with afine-meshed cover material 1310. The cover material 1310 transmits airand moisture, and the moisture absorbent 1309 does not transmit them.Note that the hermetically-sealed space 1308 may be filled with a raregas such as nitrogen or argon, and can also be filed with an inert resinor liquid.

An input terminal portion 1311 for transmitting signals to the pixelportion 1302 and the like is provided over the substrate 1301. Signalssuch as video signals are transmitted to the input terminal portion 1311through an FPC (Flexible Printed Circuit) 1312. At the input terminalportion 1311, wirings formed over the substrate 1301 are electricallyconnected to wirings provided in the FPC 1312 with the use of a resin inwhich conductors are dispersed (an anisotropic conductive resin: ACF).

A driver circuit for inputting signals to the pixel portion 1302 may beformed over the same substrate 1301 as the pixel portion 1302.Alternatively, the driver circuit for inputting signals to the pixelportion 1302 may be formed in an IC chip to be connected onto thesubstrate 1301 by COG (Chip On Glass), or the IC chip may be disposed onthe substrate 1301 by TAB (Tape Automated Bonding) or a printed board.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 19

The present invention can be applied to a display module where a circuitfor inputting signals to a panel is mounted on the panel.

FIG. 54 shows a display module where a panel 1900 is combined with acircuit board 1904. Although a controller 1905, a signal dividingcircuit 1906, and the like are formed over the circuit board 1904 inFIG. 54, circuits formed over the circuit board 1904 are not limited tothese. Any circuit which can generate signals for controlling the panelmay be formed.

Signals outputted from these circuits formed over the circuit board 1904are inputted into the panel 1900 through connecting wirings 1907.

The panel 1900 includes a pixel portion 1901, a source driver 1902, anda gate driver 1903. A structure of the panel 1900 may be similar to thatdescribed in any one of Embodiment Modes 9 to 12. Although the sourcedriver 1902 and the gate driver 1903 are formed over the same substrateas the pixel portion 1901 in FIG. 54, the display module of the presentinvention is not limited to this. Such a structure may also be employedthat only the gate driver 1903 is formed over the same substrate as thepixel portion 1901, while the source driver is formed over a circuitboard. Alternatively, both of the source driver and the gate driver maybe formed over a circuit board.

Display portions of various electronic apparatuses can be formed byincorporating such a display module.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 20

The present invention can be applied to various electronic apparatuses.The electronic apparatuses include a camera (e.g., a video camera or adigital camera), a projector, a head-mounted display (a goggle display),a navigation system, a car stereo, a personal computer, a game machine,a portable information terminal (e.g., a mobile computer, a portablephone, or an electronic book), an image reproducing device provided witha recording medium, and the like. As an example of the image reproducingdevice provided with a recording medium, a device for reproducing arecording medium such as a digital versatile disk (DVD) and having adisplay portion for displaying the reproduced image can be given. FIGS.55A to 55D show examples of the electronic apparatuses.

FIG. 55A shows a laptop computer, which includes a main body 911, ahousing 912, a display portion 913, a keyboard 914, an externalconnecting port 915, a pointing device 916, and the like. The presentinvention is applied to the display portion 913. With the presentinvention, power consumption of the display portion can be reduced.

FIG. 55B shows an image reproducing device provided with a recordingmedium (specifically, a DVD reproducing device), which includes a mainbody 921, a housing 922, a first display portion 923, a second displayportion 924, a recording medium (e.g., DVD) reading portion 925, anoperating key 926, a speaker portion 927, and the like. The firstdisplay portion 923 mainly displays image data, while the second displayportion 924 mainly displays text data. The present invention is appliedto the first display portion 923 and the second display portion 924.With the present invention, power consumption of the display portion canbe reduced.

FIG. 55C shows a portable phone, which includes a main body 931, anaudio output portion 932, an audio input portion 933, a display portion934, operating switches 935, an antenna 936, and the like. The presentinvention is applied to the display portion 934. With the presentinvention, power consumption of the display portion can be reduced.

FIG. 55D shows a camera, which includes a main body 941, a displayportion 942, a housing 943, an external connecting port 944, a remotecontrolling portion 945, an image receiving portion 946, a battery 947,an audio input portion 948, operating keys 949, and the like. Thepresent invention is applied to the display portion 942. With thepresent invention, power consumption of the display portion can bereduced.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

Embodiment Mode 21

This embodiment mode describes application examples using a displaypanel in which a display device employing the pixel structure of thepresent invention is used in a display portion, with reference to thedrawings of application modes. The display panel in which the displaydevice employing the pixel structure of the present invention is used inthe display portion can also be incorporated in a mobile object, aconstruction product, or the like.

As examples of the display panel in which the display device employingthe pixel structure of the present invention is used in the displayportion, mobile objects with a display device incorporated are shown inFIGS. 56A and 56B. FIG. 56A shows an example of a mobile object with adisplay device incorporated, in which a display panel 9702 is used in aglass of a glass door on a main body 9701 of a train vehicle. Thedisplay panel 9702 in which the display device employing the pixelstructure of the present invention is used in the display portion shownin FIG. 56A can easily switch an image displayed on the display portionby an outside signal. Therefore, by switching an image on the displaypanel for each time zone when the category of passengers is changed,advertising effectiveness can be further expected.

The display panel in which the display device employing the pixelstructure of the present invention is used in the display portion can benot only applied to the glass door on the main body of the train vehicleas shown in FIG. 56A, but also applied to any place by changing theshape. One example thereof is shown in FIG. 56B.

FIG. 56B shows the interior of a train vehicle. In FIG. 56B, in additionto the display panel 9702 in the glass door shown in FIG. 56A, a displaypanel 9703 provided for a glass window and a display panel 9704 hangingfrom the ceiling are shown. Since the display panel 9703 having thepixel structure of the present invention includes a self-luminousdisplay element, an outside view can also be seen from the train byperforming no display at times other than rush hour, whereas anadvertising image is displayed at rush hour. Further, the display panel9704 having the pixel structure of the present invention can display animage while bending the display panel itself by being provided with aswitching element such as an organic transistor over a film substrateand driving a self-luminous display element.

Further, as an application example of a mobile object which incorporatesthe display device using the display panel in which the display deviceemploying the pixel structure of the present invention is used in thedisplay portion, another application mode is described with reference toFIG. 57.

As one example of the display panel in which the display deviceemploying the pixel structure of the present invention is used in thedisplay portion, a mobile object with a display device incorporated isshown in FIG. 57. FIG. 57 shows an example in which a display panel 9902is attached to be incorporated into an automobile body 9901, as anexample of the mobile object with the display device incorporated. Thedisplay panel 9902 in which the display device employing the pixelstructure of the present invention is used in the display portion isattached to be incorporated into the automobile body, and has a functionof on-demand display of operation of the automobile body or informationinputted from inside or outside the automobile body, and further has afunction of navigation to a destination.

Note that the display panel in which the display device employing thepixel structure of the present invention is used in the display portioncan be not only applied to the front portion of the automobile bodyshown in FIG. 57, but also applied to any place such as a glass windowor a door, by changing the shape.

Further, as an application example of a mobile object which incorporatesthe display device using the display panel in which the display deviceemploying the pixel structure of the present invention is used in thedisplay portion, another application mode is described with reference toFIGS. 58A and 58B.

As one example of the display panel in which the display deviceemploying the pixel structure of the present invention is used in thedisplay portion, a mobile object with a display device incorporated isshown in FIGS. 58A and 58B. FIG. 58A shows an example in which a displaypanel 10102 is attached to be incorporated in a ceiling portion in acabin inside a plane body 10101, as an example of the mobile object withthe display device incorporated. The display panel 10102 in which thedisplay device employing the pixel structure of the present invention isused in the display portion is attached to be incorporated to the planebody 10101 by hinges 10103. The hinges 10103 stretch or fold, therebypassengers can see the display panel 10102. The display panel 10102 candisplay information and can be utilized as advertising or entertainmentmeans when it is operated by passengers. Further, with the display panel10102 stored in the plane body 10101 by folding the hinges as shown inFIG. 58B, safety in takeoff and landing can be improved. Further, bylighting a display element of the display panel in case of emergency,the display panel 10102 can also be used as a sign lighting of the planebody 10101.

Note that the display panel in which the display device employing thepixel structure of the present invention is used in the display portioncan be not only applied to the ceiling portion of the plane body 10101shown in FIGS. 58A and 58B, but also applied to any place such as a seator a door, by changing the shape. For example, by providing the displaypanel on the back side of a seat in front of your seat, the displaypanel may be operated and seen.

Although the train vehicle body, the automobile body, and the plane bodyare described as the examples of the mobile object in this embodimentmode, the present invention is not limited to these, and the presentinvention can be applied widely, such as to a two-wheeled motor vehicle,a four-wheeled motor vehicle (including an automobile, a bus, and thelike), a train (including a monorail, a railway, and the like), or boatsand ships. By employing the display panel in which the pixel structureof the present invention is used in the display portion, reduction insize and lower power consumption of the display panel can be achievedand a mobile object having a display medium with good operation can beprovided. Further, in particular, display on the display panel insidethe mobile object can be easily switched all at once by an externalsignal, thereby the display panel is very useful as an advertisingdisplay panel for an unspecified majority of customers or an informationdisplay board in disasters.

As an application example of using the display panel in which thedisplay device employing the pixel structure of the present invention isused in the display portion, an application mode of a constructionproduct is described with reference to FIG. 59.

In FIG. 59, as the display panel in which the display device employingthe pixel structure of the present invention is used in the displayportion, a display panel which can display an image while bending thedisplay panel itself by being provided with a switching element such asan organic transistor over a film substrate and driving a self-luminousdisplay element is used, and an application example thereof isdescribed. In FIG. 59, such a display panel is provided on a curvedsurface of a columnar object provided out of doors, such as anelectrical pole as a construction product, and a structure in which adisplay panel 9802 is provided for an electrical pole 9801 as aconstruction product is here described.

The display panel 9802 shown in FIG. 59 is disposed at a position whichis near the middle of height of the electrical pole and is higher than ahuman view point. Then, by seeing the display panel from a mobile object9803, an image on the display panel 9802 can be recognized. With anoutdoor forest of electrical poles or the like, by displaying the sameimage on the display panels 9802 provided for them, information and/oradvertising display can be seen. The display panels 9802 provided forthe electrical poles 9801 in FIG. 59 can easily display the same imageby the use of an external signal, thereby very effective informationdisplay and advertising effectiveness can be expected. Further, byproviding a self-luminous display element as a display element, thedisplay panel of the present invention is useful as a display mediumwhich is highly visible even in the night.

Further, as an application example of using the display panel in whichthe display device employing the pixel structure of the presentinvention is used in the display portion, another application mode of aconstruction product is described with reference to FIG. 60.

FIG. 60 shows an application example of the display panel in which thedisplay device employing the pixel structure of the present invention isused in the display portion. As one example of an object with a displaydevice incorporated, FIG. 60 shows an example in which a display panel10002 is attached to be incorporated on a side wall inside a unit bath10001. The display panel 10002 in which the display device employing thepixel structure of the present invention is used in the display portionis attached to be incorporated to the unit bath 10001. Those who bathecan see the display panel 10002. The display panel 10002 can displayinformation and can be utilized as advertising or entertainment meanswhen it is operated by those who bathe.

Note that the display panel in which the display device employing thepixel structure of the present invention is used in the display portioncan be not only applied to the side wall of the unit bath 10001 shown inFIG. 60, but also applied to any place such as a part of a mirror or abathtub itself, by changing the shape.

FIG. 61 shows an example in which a television set including a largedisplay portion is provided inside a construction product. FIG. 61includes a housing 8010, a display portion 8011, a remote-control unit8012 which is an operation portion, speaker portions 8013, and the like.The display panel in which the display device employing the pixelstructure of the present invention is used in the display portion isapplied to the display portion 8011. The television set shown in FIG. 61which is incorporated into a building, as a wall-hanging television set,can be disposed without requiring a large space.

Although the electrical pole as a columnar object, the unit bath, andthe like are described as the examples of the construction product inthis embodiment mode, the present invention is not limited to these aslong as it is a construction product capable of including a displaypanel. By employing the display panel in which the pixel structure ofthe present invention is used in the display portion, reduction in sizeand lower power consumption of the display panel can be achieved and aconstruction product having a display medium with good operation can beprovided.

Note that this embodiment mode can be implemented freely combining withany description in the other embodiment modes of this specification.Further, the descriptions in this embodiment mode can be freely combinedto be implemented.

This application is based on Japanese Patent Application Serial No.2006-155460 filed in Japan Patent Office on 2, Jun. 2006, the entirecontents of which are hereby incorporated by reference.

1. An electronic device comprising: a first wiring; a second wiring; athird wiring; a first capacitor electrically connected to the firstwiring; a second capacitor electrically connected to the second wiring;a first transistor including a gate terminal, a first terminal and asecond terminal, the gate terminal of the first transistor beingelectrically connected to the first capacitor, the first terminal of thefirst transistor being electrically connected to the second capacitor,and the second terminal of the first transistor being electricallyconnected to the third wiring; and a second transistor including a gateterminal, a first terminal and a second terminal, the gate terminal ofthe second transistor being electrically connected to the secondcapacitor, the first terminal of the second transistor beingelectrically connected to the first capacitor, and the second terminalof the second transistor being electrically connected to the thirdwiring.
 2. An electronic device according to claim 1, wherein the gateterminal of the first transistor is electrically connected to the firstwiring through the first capacitor.
 3. An electronic device according toclaim 1, wherein the gate terminal of the second transistor iselectrically connected to the second wiring through the secondcapacitor.
 4. An electronic device according to claim 1, wherein thefirst terminal of the first transistor and the gate terminal of thesecond transistor are connected to a same node.
 5. An electronic deviceaccording to claim 1, wherein the first terminal of the first transistoris electrically connected to the second wiring through the secondcapacitor.
 6. An electronic device according to claim 1, wherein thegate terminal of the second transistor is electrically connected to thesecond wiring through the second capacitor.
 7. An electronic deviceaccording to claim 1, wherein the first transistor and the secondtransistor have a same conductivity type.
 8. An electronic deviceaccording to claim 1, wherein the first and the second transistorcomprise at least one of indium, gallium, zinc and oxygen.
 9. Anelectronic device according to claim 1, wherein the electronic device isone of a computer, a game machine, an image reproducing device, aportable phone, a camera, a projector, a head-mounted display, and adisplay panel.
 10. An electronic device comprising: a first wiring; asecond wiring; a third wiring; a first capacitor having a first terminalelectrically connected to the first wiring; a second capacitor having afirst terminal electrically connected to the second wiring; a firsttransistor including a gate terminal, a first terminal and a secondterminal, the gate terminal of the first transistor being electricallyconnected to a second terminal of the first capacitor, the firstterminal of the first transistor being electrically connected to asecond terminal of the second capacitor, and the second terminal of thefirst transistor being electrically connected to the third wiring; and asecond transistor including a gate terminal, a first terminal and asecond terminal, the gate terminal of the second transistor beingelectrically connected to the second terminal of the second capacitor,the first terminal of the second transistor being electrically connectedto the second terminal of the first capacitor, and the second terminalof the second transistor being electrically connected to the thirdwiring.
 11. An electronic device according to claim 10, wherein thefirst transistor and the second transistor have a same conductivitytype.
 12. An electronic device according to claim 10, wherein the firstand the second transistor comprise at least one of indium, gallium, zincand oxygen.
 13. An electronic device according to claim 10, wherein theelectronic device is one of a computer, a game machine, an imagereproducing device, a portable phone, a camera, a projector, ahead-mounted display, and a display panel.
 14. An electronic devicecomprising: a first wiring; a second wiring; a third wiring; a fourthwiring; a first capacitor electrically connected to the first wiring; asecond capacitor electrically connected to the second wiring; a firsttransistor including a gate terminal, a first terminal and a secondterminal, the gate terminal of the first transistor being electricallyconnected to the first capacitor, the first terminal of the firsttransistor being electrically connected to the second capacitor, and thesecond terminal of the first transistor being electrically connected tothe third wiring; a second transistor including a gate terminal, a firstterminal and a second terminal, the gate terminal of the secondtransistor being electrically connected to the second capacitor, thefirst terminal of the second transistor being electrically connected tothe first capacitor, and the second terminal of the second transistorbeing electrically connected to the third wiring; a third transistorincluding a gate terminal, a first terminal and a second terminal, thegate terminal of the third transistor being electrically connected tothe second capacitor, the first terminal of the third transistor beingelectrically connected to the fourth wiring, and the second terminal ofthe third transistor being electrically connected to the third wiring.15. An electronic device according to claim 14, wherein the gateterminal of the first transistor is electrically connected to the firstwiring through the first capacitor.
 16. An electronic device accordingto claim 14, wherein the gate terminal of the second transistor iselectrically connected to the second wiring through the secondcapacitor.
 17. An electronic device according to claim 14, wherein thefirst terminal of the first transistor and the gate terminal of thesecond transistor are connected to a same node.
 18. An electronic deviceaccording to claim 14, wherein the first terminal of the firsttransistor is electrically connected to the second wiring through thesecond capacitor.
 19. An electronic device according to claim 14,wherein the gate terminal of the second transistor is electricallyconnected to the second wiring through the second capacitor.
 20. Anelectronic device according to claim 14, wherein the first transistorand the second transistor have a same conductivity type.
 21. Anelectronic device according to claim 14, wherein the first and thesecond transistor comprise at least one of indium, gallium, zinc andoxygen.
 22. An electronic device according to claim 14, wherein theelectronic device is one of a computer, a game machine, an imagereproducing device, a portable phone, a camera, a projector, ahead-mounted display, and a display panel.